The Classical Stefan Problem Basic Concepts, Modelling and Analysis
NORTH-HOLLAND SERIES IN
APPLIED MATHEMATICS AND MECHANICS EDITORS:
J.D. ACHENBACH Northwestern University
F. MOON Cornell University
K. SREENIVASAN Yale University
E. VAN DER GIESSEN Un ivers ityofG ron ingen
L. VAN WIJNGAARDEN Twente University of Technology
J.R. WILLIS University of Cambridge
VOLUME 45
ELSEVIER AMSTERDAM - BOSTON - HEIDELBERG - LONDON - NEW YORK - OXFORD PARIS - SAN DIEGO - SAN FRANCISCO - SINGAPORE - SYDNEY - TOKYO
The Classical Stefan Problem Basic Concepts, Modelling and Analysis
S.C. GUPTA Professor (Retd), Department of Mathematics, Indian Institute of Science, Bangalore, India
ELSEVIER 2003
AMSTERDAM - BOSTON - HEIDELBERG - LONDON - NEW YORK - OXFORD PARIS - SAN DIEGO - SAN FRANCISCO - SINGAPORE - SYDNEY - TOKYO
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Dedicated to Free Boundary Fraternity
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Contents
Chapter 1. The Stefan Problem and its Classical Formulation 1.1 1.2 1.3 1.4
Sonic, Strfaii and St(:fiili-lik(, Prolilcills Free Boimtlary Problcrris with Frcc Bouiitlaries of Cotliiiiciisioii-two Thc, Classical Stefan Prol)lciii in Oiic-tlinieiisiori and t,hc Neuniaiiii Solutiorr St(!f;iIi Prol)lcrns Classical Forrnrilatiori of ~liilt,i-tliiiic~nsio~i~~l 1.4.1 Two-Phase Stcfm prol)lciii iii iiinltiplctliriieiisioiis 1.4.2 Alt,erriate forms of the Stcfui c:oiidit,iori 1.4.3 Tlic Kirchhoff’s t,ransfoririiition 1.4.4 Boundary conditions at the fixed liountlary 1.4.5 Conditions at the free lioiiiidary 1.4.6 The classical solutiori 1.4.7 Conservation laws and the iiiotion of the melt,
1 1 18 19 23 23 28 29 30 33 34 35
Chapter 2. Thermodynamical and Metallurgical Aspects of Stefan Problems
39
2.1 Therrnodynamical Aspects 2.1.1 Microscopic arid niacroscopic models 2.1.2 Laws of classical thermodynamics 2.1.3 Some thermodynamic variables and thermal parameters 2.1.4 Equilibriuni temperature; Clapeyron’s equation 2.2 Some Metallurgical Aspects of Stefan Problems 2.2.1 Nucleation aiid superc:ooling 2.2.2 The effect of interface curvature 2.2.3 Nucleation of melting, effect of interface kinetics, aiid glassy solids 2.3 Morphological Instability of tlic Solid--Liquid Interface 2.4 Non-material Singular Surface: Generalized Stefan Condition
39 39 40 42 45 47 47 49 51 52 55
Chapter 3. Extended Classical Formulations of n-phase Stefan Problems with n 2 1
61
3.1 One-phase Problcrris 3.1.1 An extended formnlatiori of one-diineiisional one-phase problcin 3.1.2 Solidification of supercooled liquid 3.1.3 Miilt,i-diincnsioiia1 oiic.-pliasc problcms 3.2 Ext,critlctl Classical Forniu1;Ltioiis of T w o - p h a ~St.(’fiiil ~ Prol,l(mis 3.2.1 A I I cxtcntlcd forniiil;it,ioii of tho oii(~-(liiii(~iisioii~il t.wo-pliasc prohlciii 3.2.2 R’Iiilti-diiiiciisioii~L1Stcfaii pro1)lenis of classcs I1 iuitl 111 :1.2.:3 C1;issiciil St,cfiLii prol)lriiis wit,h Ir-pliascs, t!. > 2 3.2.4 Solitlificatiori wit I i t r;iiisit,ioii t c w p ( w t . i i r c raiigo vii
61 61 63 64 66 Mi GX
70 73
viii
Contents
3.3 Stefan problems with Implicit Free Boundary Conditions 3.3.1 Schatz transformations and implicit free boundary conditions 3.3.2 Unconstrained and constrained oxygen-diffusion problem (ODP)
76 77 81
Chapter 4. Stefan Problem with SupercooUng: Classical Formulation and Analysis
85
4.1 Introduction 4.2 A Phase-field Model for Solidification using Landau Ginzburg Free Energy Functional 4.3 Some Thermodynamically Consistent Phase-field and Phase Relaxation Models of Sohdification 4.4 Solidification of Supercooled Liquid Without Curvature Effect and Kinetic Undercoohng: Analysis of the Solution 4.4.1 One-dimensional one-phase sohdification of supercooled hquid (SSP) 4.4.2 Regularization of blow-up in SSP by looking at CODP 4.4.3 Analysis of problems with changes in the initial and boundary conditions in SSP 4.5 Analysis of Supercooled Stefan Problems with the Modified Gibbs Thomson Relation 4.5.1 Introduction 4.5.2 One-dimensional one-phase supercooled Stefan problems with the modified Gibbs-Thomson relation 4.5.3 One-dimensional two-phase Stefan problems with the modified Gibbs-Thomson relation 4.5.4 Multi-dimensional supercooled Stefan problems and problems with the modified Gibbs-Thomson relation 4.5.5 Weak formulation with supercooling and superheating effects
85 86 94 104 104 107 109 112 112 113 115 121 126
Chapter 5. Superheating due to Volumetric Heat Sources: The Formulation and Analysis
129
5.1 The Classical Enthalpy Formulation of a One-dimensional Problem 5.2 The Weak Solution 5.2.1 Weak solution and its relation to a classical solution 5.2.2 Structure of the mushy region in the presence of heat sources 5.3 Blow-up and Regularization
129 133 133 137 139
Chapter 6. Steady-State and Degenerate Classical Stefan Problems
142
6.1 Some Steady-state Stefan Problems 6.2 Degenerate Stefan Problems 6.2.1 Quasi-static Stefan problem and its Relation to the Hele- Shaw problem
142 143 146
Contents
ix
Chapter 7. Elliptic and Parabolic Variational Inequalities
148
7.1 Introduction 7.2 T h e Elliptic Variational Inequality 7.2.1 Definition and the basic function spaces 7.2.2 Minimization of a functional 7.2.3 T h e complementarity problem 7.2.4 Some existence and uniqueness results concerning elliptic inequalities 7.2.5 Equivalence of different inequality formulations of an obstacle problem of the string 7.3 T h e Parabolic Variational Inequality 7.3.1 Formulation in appropriate spaces 7.4 Some Variational Inequality Formulations of Classical Stefan Problems 7.4.1 One-phase Stefan problems 7.4.2 A Stefan problem with a quasi-variational inequality formulation 7.4.3 T h e variational inequality formulation of a two-phase Stefan problem
148 149 149 152 153 156
Chapter 8. The Hyperbolic Stefan Problem 8.1 Introduction 8.1.1 Relaxation time and relaxation models 8.2 Model I: Hyperbolic Stefan Problem with Temperature Continuity at the Interface 8.2.1 The mathematical formulation 8.2.2 Some existence, uniqueness and well-posedness results 8.3 Model II: Formulation with Temperature Discontinuity at the Interface 8.3.1 T h e mathematical formulation 8.3.2 T h e existence and uniqueness of the solution and its convergence as r ^ 0 8.4 Model III: Delay in the Response of Energy to Latent and Sensible Heats 8.4.1 T h e classical and the weak formulations
166 170 170 174 174 187 191
196 196 197 199 199 203 207 207 209 216 216
Chapter 9. Inverse Stefan Problems
224
9.1 Introduction 9.2 Well-posedness of the solution 9.2.1 Approximate solutions 9.3 Regularization 9.3.1 T h e regularizing operator and generalized discrepancy principle 9.3.2 T h e generalized inverse 9.3.3 Regularization methods 9.3.4 R a t e of convergence of a regularization method 9.4 Determination of Unknown Parameters in Inverse Stefan Problems 9.4.1 Unknown parameters in the one-phase Stefan problems 9.4.2 Determination of unknown parameters in the two-phase Stefan problems
224 225 229 231 231 234 235 243 244 245 248
X
Contents
9.5 Regularization of Inverse Heat Conduction Problems by Imposing Suitable Restrictions on the solution 9.6 Regularization of Inverse Stefan Problems Formulated as Equations in the form of Convolution Integrals 9.7 Inverse Stefan Problems Formulated as Defect Minimization Problems
249 252 257
Chapter 10. Analysis of the Classical Solutions of Stefan Problems
271
10.1 One-dimensional One-phase Stefan Problems 10.1.1 Analysis using integral equation formulations 10.1.2 Infinite differentiability and analyticity of the free boundary 10.1.3 Unilateral boundary conditions on the fixed boundary: Analysis using finite-difference schemes 10.1.4 Cauchy-type free boundary conditions 10.1.5 Existence of self-similar solutions of some Stefan problems 10.1.6 T h e effect of density change 10.2 One-dimensional Two-phase Stefan Problems 10.2.1 Existence, uniqueness and stability results 10.2.2 Differentiabihty and analyticity of the free boundary in the one-dimensional two-phase Stefan problems 10.2.3 One-dimensional n-phase Stefan problems with n > 2 10.3 Analysis of the Classical Solutions of Multi-dimensional Stefan Problems 10.3.1 Existence and uniqueness results valid for a short time 10.3.2 Existence of the classical solution on an arbitrary time interval
272 272 286
Chapter 11. Regularity of the Weak Solutions of Some Stefan Problems
291 295 298 299 300 300 311 312 315 315 321
322
11.1 Regularity of the Weak solutions of One-dimensional Stefan Problems 11.2 Regularity of the Weak solutions of Multi-dimensional Stefan Problems 11.2.1 T h e weak solutions of some two-phase Stefan problems in R", n > 1 11.2.2 Regularity of the weak solutions of one-phase Stefan problems in W\ n > 1
322 329 329
Appendix A. Preliminaries
338
Appendix B. Some Function Spaces and Norms
345
Appendix C. Fixed Point Theorems and Maximum Principles
349
333
Appendix D. Sobolev Spaces
351
Bibliography
355
Captions for Figures
381
Subject Index
383
List of Symbols Some symbols and abbreviations which are common throughout the book are given below. They have also been explained in the text, occasionally, but not every time they occur. In addition to these symbols, several other symbols have been used in the text, and they have been explained at the places they occur. The parameters occurring in the equations could be dimensionless quantities or may have dimensions. At those places where the equations are in the dimensionless form, the method of dimensionalization has been mentioned. For the notations used for the function spaces, the reader is referred to Appendices A—D. The same have been explained in the text, occasionally. Standard notations have been used for the numbering of equations, figures, definitions and propositions. For example, (7.2.7) refers to the seventh equation in the second section of chapter 7. c(x, t)
Concentration,
[kg/m^]
C
Specific heat,
Cy
Specific heat at constant volume
[Jtcg-'^K'^]
Cp
Specific heat at constant pressure
C
Heat capacity,
e
Specific internal energy,
[JK~^/nr'] [Jkg~^]\ also used for energy per unit volume
(indicated in the text) H
Enthalpy,
[J]; also enthalpy per unit volume (indicated in the text)
h
Specific enthalpy,
ht
Heat transfer coefficient,
k
Thermal diffusivity,
K
Thermal conductivity in the isotropic case,
Kij
[Jkg''^] \WK'^/m^]
[m^s~^] [Jrn"^s~^K~^]
Thermal conductivity coefficients in an anisotropic case; z = 1, 2, 3 and J = 1,2,3
Kc
Mean curvature of the free boundary,
[m~^]
I
Latent heat of fusion,
Im I
Latent heat per unit mole, I + {CL - Cs)T,,,
7?"
11 > 1, r(uil n-dimensional space, R or R^ used for real line
S{t)
X ^ S[t) (:r = S[y,zj)), equation of the phas(^-change boundary in one-dimension (three-diuKnisions)
[Jkg'^] [J/A; rrwl]
s
Specific entropy,
[Jkg
'A' ^]
S
Entropy,
[JK-^]
t
real time,
[.s]
T
Temperatnre,
Tm
Ideal equilibrium melting/freezing temperature, also taken as 0 or 1
T,^,
Equilibrium phase change temperature in supercooling/superheating
KTJ
Molar volume,
ft
unit normal vector
[°K]
[m'^/kmol]
Subscripts (L, S, M)
Stand for liquid, solid and mushy regions, respectively
2 = 1,2
quantities in the two phases
Greek symbols p
Density,
[kg/m'^]
a
Surface tension,
[Nm~^]
Some other symbols
/
Differentiation with respect to the argument Time derivative
v/ V2
Gradiant of a scalar function Laplacian operator
erf(x)
Error function
erfc(x)
l-erf(x)
Abbreviations meas (A)
measure of the set A
CEF
Classical enthalpy formulation
CES
Classical enthalpy solution
CO D P
Constrained oxygen-diffusion problem
CSS
Classical St,(^fan solution
HSP
Hele-Shaw problem
ODP
Oxygen-diffusion problem
QSSP
Quasi steady-state problem
SPF
Standard phase-field model
SSP
Supercooled Stefan problem
UODP
Unconstrained O D P
WS
Weak solution
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Preface This volume emphasises studies related to classical Stefan problems. The term "Stefan problem" is generally used for heat transfer problems with phase-changes such as from the hquid to the solid. Stefan problems have some characteristics t h a t are typical of them, but certain problems arising in fields such as mathematical physics and engineering also exhibit characteristics similar to them. The term "classical" distinguishes the formulation of these problems from their weak formulation, in which the solution need not possess classical derivatives. Under suitable assumptions, a weak solution could be as good as a classical solution. In hyperbolic Stefan problems, the characteristic features of Stefan problems are present but unlike in Stefan problems, discontinuous solutions are allowed because of the hyperbolic nature of the heat equation. The numerical solutions of inverse Stefan problems, and the analysis of direct Stefan problems are so integrated t h a t it is difficult to discuss one without referring to the other. So no strict line of demarcation can be identified between a classical Stefan problem and other similar problems. On the other hand, including every related problem in the domain of classical Stefan problem would require several volumes for their description. A suitable compromise has to be made. The basic concepts, modehing, and analysis of the classical Stefan problems have been extensively investigated and there seems to be a need to report the results at one place. This book attempts to answer t h a t need. Within the framework of the classical Stefan problem with the emphasis on the basic concepts, modelling and analysis, I have tried to include some weak solutions and analytical and numerical solutions also. The main considerations behind this are the continuity and the clarity of exposition. For example, the description of some phase-field models in Chapter 4 arose out of this need for a smooth transition between topics. In the mathematical formulation of Stefan problems, the curvature eff"ects and the kinetic condition are incorporated with the help of the modified Gibbs-Thomson relation. On the basis of some thermodynamical and metallurgical considerations, the modified Gibbs-Thomson relation can be derived, as has been done in the text, but the rigorous mathematical justification comes from the fact that this relation can be obtained by taking appropriate limits of phase-field models. Because of the unacceptability of some phase-field models due their so-called thermodynamical inconsistency, some consistent models have also been described. This completes the discussion of phase-field models in the present context. Making this volume self-contained would require reporting and deriving several results from tensor analysis, differential geometry, non-equilibrium thermodynamics, physics and functional analysis. I have chosen to enrich the text with appropriate references so as not to enlarge the scope of the book. The proofs of propositions and theorems are often lengthy and different from one another. Presenting them in a condensed way may not be of much help to the reader. Therefore only the main features of proofs and a few results have been presented to suggest the essential flavour of the theme of investigation.
However at each place, appropriate references have been cited so that inquisitive readers can follow them on their own. Each chapter begins with basic concepts, objectives and the directions in which the subject matter has grown. This is followed by reviews-in some cases quite detailed—of published works. In a work of this type, the author has to make a suitable compromise between length restrictions and understandability. I have followed my best judgement in this regard. I hope the readers will appreciate my efforts.
S.C. Gupta, Email :
Bangalore
[email protected]
Acknowledgements I gratefully acknowledge the financial support from the Department of Science and Technology, Ministry of Science and Technology, Government of India, without which it would not have been possible for me to undertake this book-writing project. The administration of the Indian Institute of Science, Bangalore, and the Department of Mathematics, provided infrastructural support for which I am extremely thankful. During my visit to the University of Oulu, Finland, in 1997, I collected lot of material relevant for this book. I sincerely thank my host. Professor Erkki Laitinen and Department of Mathematics, Oulu, for their kind hospitality. The idea of writing this book nucleated during my visit to University of Rosario, Argentina, in 1991, and fructified eleven years later. I thank Professor D.A. Tarzia, Department of Mathematics, Universidad Austral, Rosario, for sending me the bibliography prepared by him, and some reprints which I could not get from other sources. I am grateful to Professor Adimurthi, Department of Mathematics, TIER, at IISc, Bangalore, and professor A.K. Lahiri, Department of Metallurgy, Indian Institute of Science, Bangalore, for reading and responding to some portions of this book. My grateful thanks to Dr. Thomas Chacko, Professor of English and Communication Skills, Department of Management Studies, for style editing this volume with a great deal of attention. There are many old friends and well-wishers who sent reprints and preprints that proved very useful for me. Without naming them individually, I thank all of them for their selfless help. I thank the Pubhshing Editor and staff of Elsevier and the Series Editor Professor J.D. Achenbach of the book series 'Applied Mathematics and Mechanics' for their cooperation. The excellent typing in Latex and drawing of figures in computer graphics were done by Susheel Graphics, Bangalore. Without their cooperation the work would not have taken the final shape.
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Chapter 1 The Stefan Problem and its Classical Formulation 1.1
Some Stefan and Stefan-like Problems
The term Stefan problem can be best understood with the help of an example for which the reader is referred to § 1.3. Our concern in the present section is to understand the notion of a free boundary which is a typical feature of the Stefan and Stefan-like problems. Therefore this chapter begins with some examples of Stefan and Stefan-like problems which demonstrate the existence of an unknown boundary, commonly known in the literature as a 'free boundary' or a moving boundary. In the context of solidification/melting problems, with which Stefan problems are commonly associated, the free boundary is also called a phase-change boundary or a melting front or a freezing front Some authors use the term free boundary when the unknown boundary is static and moving boundary when it is time dependent. In this volume we do not make any distinction between 'free' and 'moving' boundaries. The term boundary is used for a surface also. In many of the examples considered in this section, the identification of the free boundary and the mathematical formulation of the problem are rather easy but in some of them even the identification of the free boundary is difficult. The problems given here are from various fields of mathematics, physics and engineering and demonstrate the existence of free boundaries. Our interest in examples given in this section is more on demonstrating the existence of a free boundary and its typical characteristics than justifying the formulation. Problem 1.1.1.
Steady-state heat conduction with a free boundary
Find the steady-state temperature T(x, y) satisfying the equation d^T
d''T
in an open bounded region D C R^. The boundary dD of D consists of two disjoint 1
2
The Stefan Problem and its Classical
Formulation
parts Ri and R2, i.e., dD = RiU R2, where Ri is unknown and R2 is known. On R2, the temperature is prescribed as T{x,y)\R,=f(x,y).
(1.1.2)
If f{x, y) is known throughout the plane, then one boundary conditions on Ri will be T{x,y)\R,=f{x,y),
(1.1.3)
and another boundary condition can be imposed as [Ij, v ( r - / ) | „ , =0.
(1.1.4)
T h e problem is to determine the temperature T(x, y) in D, and the unknown boundary Ri. Two boundary conditions are to be prescribed on Ri. One, because it is a boundary and one more boundary condition is required to determine an unknown boundary. Note t h a t Ri can be determined only by solving the system (1.1.1)—(1.1.4). Such an unknown boundary is called a free boundary or a moving boundary. In 'boundary value problems' of mathematical physics, the boundary of the region under consideration is completely known. Conditions (1.1.3) and (1.1.4) are called free boundary conditions and (1.1.2) is a fixed boundary condition. Although the equation (1.1.1) is linear, free boundary problems are nonlinear problems because of the nonlinearity of the boundary conditions at the free boundary. P r o b l e m 1.1.2.
S t e a d y - S t a t e PVee Surface F l o w w i t h Surface T e n s i o n
Consider a two-dimensional steady, incompressible, irrotational flow in a long channel. X and y axes are taken along the length and depth of the channel, respectively, with the b o t t o m of the channel taken as ?/ = 0 and the upper surface of water, as a free surface or a free boundary denoted by y = '^(^), where ri{x) is unknown. If u{x,y) and v{x,y) are the velocity components in x and y directions, respectively, then du dv - — h 77~ = 0 ox oy dv — ox
du —— = 0 oy
. r • • \ (equation oi contmuity) ,
(irrotationality condition) .
/i -. r\ (1.1.5)
(1.1.6)
The pressure p is given by the following Bernoulli equation [2] H = y + pl(pg) + \q\'l(2g),
(1.1.7)
where, H is the given total water head of water, p is the density, g the acceleration due to gravity and ^ i s the velocity vector. If the bottom of the channel is a rigid boundary, then v = Q, on ?/ = 0.
(1.1.8)
1.1. Some Stefan and Stefan-like Problems
3
At the free boundary y = r/(x), i / = 7/(x)+p/(p^) + |g|V(2p), p = p-a/R,
(1.1.9)
and q-n = 0.
(1.1.10)
Here, p is the atmospheric pressure (known), a is the surface tension (known) and Re is the radius of curvature of the free surface, taken as positive when the center of curvature is situated above the free surface and n is the unit outward normal to the free surface. Re can be expressed in terms of derivatives of r]{x). Free boundary has been taken as static in this problem. Equation (1.1.10) implies that the normal component of the fluid velocity at the free boundary is zero. Problem 1.1.3.
Free Surface Flow^ with Time Dependent Free Surface
Let the free surface in Problem 1.1.2. be time dependent and represented by y = r]{x,t) with ri\t=o being given. Surface tension effects will be neglected. If the velocity field ^is expressed as f=V(/>,
(1.1.11)
then from (1.1.5) and (1.1.6) it is easy to conclude that 0(x, t) satisfies the equation
§ + 0 = 0' ''W<2'<'?(^'^)-
(1-1-12)
Here, y = b{x) is the equation of the bottom of the channel. The momentum equation can be written as (cf. [3]) ^ + - gradl^p - qA cmlq = F - grad(p/p),
(1.1.13)
F represents body forces. On substituting g* from (1.1.11) in (1.1.13) and integrating with respect to x, we obtain ^
+ ^|V0|' + gy-vpip = m .
(1-1-14)
provided the density is taken as constant and the gravitational field is the only force field. The arbitrary function ip{t) can be absorbed in 0(t) and (1.1.14) becomes ^
+ ^|V<^P +fl2/+ p/p = 0.
(1.1.15)
If y = b{x) is taken as a rigid boundary, then -^ = 0 on y = b{x),
(1.1.16)
4
The Stefan Problem and its Classical Formulation
where n denotes the unit outward normal to y = b(x). On the unknown free boundary y = rj{x,t)^ the two conditions are given by ^
+ ^ | V 0 p + 5r,(x,f)+p/p = O,
(1.1.17)
and
where V is the velocity of the free boundary and n is the unit outward drawn normal on it. Equation (1.1.18) can be expressed in terms of quantities already defined. Let, F(x, y,t)=y-
r](x, t) = 0.
(1.1.19)
Then dF OF dF DF = —dx + —dy + —dt = 0,
(1.1.20)
or
From (1.1.18) and (1.1.21), we get f
=
|^|VFH(V0-n)|VF| V(/)-i|^j|VFH(V0.VF)
dri^_d^dri^d^ dt dx dx
(1.1.22)
^ ^ 23) dy
Equation (1.1.12) is to be solved using the boundary conditions (1.1.17) and (1.1.23), the fixed boundary condition (1.1.16) and the prescribed ri{x, t) at t = 0. In this problem the velocity of the fluid is not time dependent but the free boundary is time dependent. Such problems are called quasi-steady state free boundary problems or degenerate free boundary problems. We shall see later that the term degenerate free boundary problem is used for other types of problems also. Linearization of the above problem Let y = i/o be the flat upper surface of water. When the deviation of the free boundary from the flat surface is small, i.e., if \dr]/dx\ < < 1, then the above problem can be linearized as follows. Let y = ri{x,t) =yQ +ef{xA),
(j) = eu,£«l.
(1.1.24)
1.1. Some Stefan and Stefan-like
Problems
5
£f{x,t) can be considered as the disturbance at the flat surface y = y^. Substituting (1.1.24) in (1.1.12), (1.1.16), (1.1.17) and (1.1.23), and in the changed equations retaining only hnear terms in e and dropping higher order terms of e, we obtain
V'u = 0 , b{x)
< !/ < 2/0,
du = 0 on ^ y --- b{x), dn du
(1.1.26)
9f = 0 on y = yo^
du dy
df dt
(1.1.25)
(1.1.27)
on y = yo-
(1.1.28)
In (1.1.27), we have taken p = ~ pgyo which comes from the contribution of the zeroth order terms in (1.1.17). On ehminating / from (1.1.27) and (1.1.28), we obtain d'^u du ^ _ _ + 3 _ = 0
on
y = y,.
. ^ ^^, (1.1.29)
Since y = yo is not a free boundary, only one boundary condition is to be prescribed on it. For further information about the Problems 1.1.2. and 1.1.3., see ([3, 4, 5]). P r o b l e m 1.1.4.
A P r o b l e m of R e p r o d u c t i v e Toxic M a s s Diffusion
Let u{x, t) be the concentration of a toxic mass which is diflFusing in a region Q, where n = {xe
R:0<x
(1.1.30)
If the concentration exceeds a certain value Uy in a portion of ^2, then it is called a toxic region. Let the reproduction rate of toxic mass in the toxic region be P and in the nontoxic region aP, 0 < a < 1. T h e toxic and non-toxic regions are separated by a surface S, where S - {{x, t) enu\x
= (pit) , u = Uy}
(1.1.31)
and fit. = {{x,t)\xe
n,
0
t,}.
(1.1.32)
u{x, t) and the free boundary x = (f){t) are to be obtained by solving the following system of equations. For u > Uy ut - u^x + d^ + dyu = P, (x, 0 G 0^,, , 0 < X < 0(t), 0 < ^ < t*.
(1.1.33)
For u < Uy Ut - u^,^-h do ^ diu = aP,
(x,t)enu,
(t){t) < x < 1, 0 < t < t^.
(1.1.34)
6
The Stefan Problem and its Classical
Formulation
The term
= Ux((l){t)-\',t),
(continuity of mass
flux)
(1.1.35)
and u[(t){t),t) =Uy.
(1.1.36)
It may be noted that the velocity of the free boundary is not exphcitly occurring in (1.1.35) which was so in Problems 1.1.2 and 1.1.3. The boundary conditions at the free boundary in which the velocity of the free boundary is not occurring explicitly are known as implicit free boundary conditions. Equations (1.1.33)—(1.1.36) are to be supplemented with a suitable initial condition at t = 0 and with boundary conditions at the fixed boundaries x = 0 and x = 1. Some results on the existence of solution of the above problem and the regularity of the free boundary can be found in [6]. P r o b l e m 1.1.5.
Gas Flow Through Porous Media
The equation of state for an isentropic (constant entropy) flow of an ideal gas in a homogeneous porous media is given by [7] p(x, 2/) = POP" > 0, where p(x, y) is the density and p(x, y) is the pressure, constant. The conservation of mass gives
(1.1.37) po G R^ and a G (0,1] are
div(pV) = - 7 | ^ ,
(1.1.38)
where 7 is the porosity of the medium. According to Darcy's law [8], the velocity V of the gas flow in a porous medium is given by V = ~{P/r])gT8idp,
(1.1.39)
P G R^ is the permeability of the medium and r] G R^ is the viscosity of the gas. V and p can be efiminated from (1.1.38) and (1.1.39) and we obtain
| where m = I + l/a.
Z ,
V ^ ) , p>0,
(1.1.40)
The diffusion is called 'fast' if rn > 1, and 'slow' if ni < 1.
By suitably choosing the time a n d / o r length scales, the following equation can be obtained from (1.1.40). ^
= V^(p'"), p > 0 .
(1.1.41)
1.1. Some Stefan and Stefan-like
Problems
7
If a ^ (O^l]^ then m G [2,oo). Equation (1.1.41) is the porous media equation and it arises also in other contexts such as population dynamics and plasma physics [7j. In order to calculate the mass flux of the gas, the r.h.s. of (1.1.41) can be written as V^p"^) = div(m/?^-^ gradp),
(1.1.42)
jjip-m-i gj-ajp is the mass flux and mp^~^ can be taken as diffusivity. Diff'usivity vanishes as the density p tends to zero. Therefore (1.1.41) is a nonlinear degenerate equation in the neighbourhood of any point where p = 0 but is non-degenerate and uniformly parabolic (see [9] and (7.3.26)) in the neighbourhood of any point at which p is away from zero. Such problems are called degenerate parabolic-elliptic problems. An important consequence of nonlinear degeneracy is t h a t there is a finite speed of propagation of a disturbance from rest which is in contrast to the parabolic heat equation in which the speed of heat propagation is infinite. The finite speed of propagation may give rise to waiting time solutions. Equation (1.1.41) is to be supplemented with an initial condition if the region is infinite and with both initial and boundary conditions if the region considered is finite. T h e existence of a free boundary in such problems can be illustrated with the help of the following example. Let - ^ = V^(p^),
- o o < X < oo, t > 0,
(1.1.43)
> 0 for x G i?/ = (ai, 02), —00 < ai < a2 < 00 (1.1.44)
!
- 0 for x G
R\Ri.
This problem, generally, does not have a classical solution. T h e classical solution of a problem can be roughly stated to be a solution in which the dependent variable possesses continuous derivatives of the order required in the problem formulation. The mathematical definition of a classical solution will be discussed later but at present it would suffice to state t h a t the solution p[x,t) of (1.1.43) and (1.1.44) may not possess the required continuous derivatives. For t > 0, gas will be diff"using to the right of x = a2 and to the left oix = ai and thus giving rise to two moving boundaries x = Si[t),i = 1,2. Let Si be moving towards + 0 0 and 5*2 moving towards —00. Using a weak formulation of the above problem in (1.1.43) and (1.1.44), several interesting results on the behavior oi Si{t), z = 1, 2 have been obtained in [10] and [11]. T h e following proposition indicates that in some cases, the interface Si{t) starts moving only after an elapse of time V > 0. P r o p o s i t i o n 1.1.1. There exist numbers t* G [0, +00) for z = 1,2 such that Si{t) is strictly monotone for t G (t*, +cx)) and S^{t)=a,, If t* > 0, then Si{t),
z = 1,2 for t G [0,t*].
(1.1.45)
z = 1, 2 remain stationary for t* units of time [11].
In this case t* is called a waiting time. It has been proved in [11] that the interface is Holder continuous under certain conditions and if the interface is in motion, then one
8
The Stefan Problem and its Classical Formulation
expects it to move with the velocity V of the gas, and ^
W^,(x, t), z = 1, 2; W = - ^ ^ p " ^ - ^
= - lim
(1.1.46)
The hmits in (1.1.46) are taken as x approaches the boundary of the region from within the region in which p{x,t) > 0. Equation (1.1.46) can also be obtained from the mass balance condition at the interface which states that the jump in the density at the interface multiplied by the velocity of the interface is equal to the jump in the mass flux across the interface. In a problem symmetrical with respect to x, it is sufficient to consider the region 0 < X < oo with a single interface x = Si{t) and ai = a2 > 0. Si{t) should satisfy (1.1.46) for z = 1, and another condition to determine the unknown Si{t) may be prescribed as Slit)
a,
fp{x,t)dx=
f p{x,0)dx.
0
(1.1.47)
0
Conditions of the type (1.1.47) are called non-local boundary conditions at the free boundary. X = 0 is now a fixed boundary and the boundary condition on it is given by dp dx
= 0.
(1.1.48)
x=0
Note that we have two conditions prescribed at the free boundary, viz., (1.1.46) and (1.1.47). Problem 1.1.6.
Shock Propagation
The solution of Burger's equation (1.1.49) with boundary condition (1.1.50) has been discussed in [12]. Uy + uu^ = Uy+ 2 a~^^^^ ^ ^' ^^' ^^ ^ ^^' ^ - 0-
(1.1.49)
On ^ = 0, u{x,y)\y^, = f{x),
xeR,
(1.1.50)
0 < X < 1,
(1.1.51)
where
fix)
=
1, x < 0 , 1 -X,
0, x > 1. The characteristic equations of (1.1.49) in parametric form in terms of a parameter t are given by dx dy . du ^ — - 0, - ^ = 1, and — = 0. dt dt dt
_ ^ ^. (1.1.52)
1.1. Some Stefan and Stefan-like
Problems
Let x{s, 0) = s, y{s, 0) = 0 and ^(5, 0) =
f{s).
The solution of (1.1.52) is given by ' 5 + t, s < 0, x{s,t)
( l - s ) t + s, 0 < 5 < 1, ;
••
S,
y = t
(1.1.53)
5 > 1. 1,
s < 0,
( 1 - s ) , 0 < s < 1,
u{s^ t) •
0,
(1.1.54)
s> 1.
The characteristic curves and u{x^y) are shown in Fig. 1.1.1. It is clear from the figure t h a t the characteristic curves intersect in the region x > 1, y > 1 and therefore u{x,t) is not defined (is not single valued) in this region. u{x,y) defined by (1.1.54) ceases to be the classical solution of (1.1.49) in the region x > 1, y > I and (1.1.49) is satisfied in this region only in the weak sense. It may be noted t h a t the solution u{x, y) could be discontinuous in some portion of the region even if / ( x ) is a continuous and differentiable function throughout the region. If (1.1.49) is written as
dP_ aO dy
dx
0,
(1.1.55)
then it can be shown (cf. [12]) that the weak solution is discontinuous across a curve which is called a shock and whose equation is given by
[P]dx - [Q]dy = 0.
(1.1.56)
The notation [/] denotes the difference between the limits of a function / as the shock or a surface of discontinuity of / is approached from both sides. Conditions of the type (1.1.56) are called Rankine-Hugoniot conditions. In the present problem [P] = 1 and [Q] = 1/2, and therefore the shock is given by y = 2x + d. The constant d can be determined if we know the point from which the shock is emanating, for example, in the present problem this point is (1.1) (see Fig. 1.1.2.). Shock is the oldest form of the free boundary and its origin lies in the study of gas dynamics [13]. If (1.1.49) is multiplied by u'^.n > 1, then P and Q in (1.1.55) will change. This change will result in the non-uniqueness of the shock and infinitely many shocks can be obtained. For nonlinear hyperbolic equations of the type (1.1.49), further information in the form of physical arguments involving stability, entropy, dissipation or continuous dependence on the initial d a t a is needed to ensure uniqueness (cf. [14, 15]). There are some important differences between the present problem and the free boundary Problems 1.1.1. to 1.1.5. In the previous five problems, the free boundaries can be
10
The Stefan Problem and its Classical Formulation y4
0
Fig. 1.1.1. Characteristic curves and u{x,y)
X
Fig. 1.1.2. Geometry of the shock
identified without much difficulty and the boundary conditions on them can be imposed after some thought. It is a different matter that the classical solution may or may not exist. In the solution of equations (1.1.49)—(1.1.50), there is no indication of a free boundary. It is only through the construction of the solution that we come to know about the magnitudes of jumps in P and Q. The equation (1.1.56) can be obtained only through the weak solution and not through the classical approach, which was followed in other problems. Problem 1.1.7. Free Boundary Associated with a Frictional Oscillator Problem This interesting free boundary problem has been reported in [15]. As shown in Fig. 1.1.3., a block of mass m rests on a conveyer belt moving with a constant velocity V. The forces acting on the mass are: (1) spring force with a spring modulus 5^, (2) prescribed force F{t) assumed to be sufficiently smooth, and (3) Coulomb frictional force with coefficient of friction /i. The motion of the block with mass m is governed by the equation dx sgn
-V
\+S,
m.
(1.1.57)
where sgn{z)
1 if 2 > 0,
-1 if 2 < 0 ,
(1.1.58)
and g is the acceleration due to gravity. At time t = 0, x and dx/dt are prescribed. Both analytical and numerical solutions of (1.1.57) are extremely difficult as at each instant of time information is required whether dx/dt > y, = y, or < V. The unknown instants of time t = t^, i = 1,2,... or the points x^ = x[ti), z = 1, 2, at which dx/dt = V can be regarded as free boundaries.
1.1. Some Stefan and Stefan-like
Problems
11
When dx/dt = V, d^x/dt'^ = 0. The second derivative becomes discontinuous dX t = t^. Once dx/dt becomes V at ^ = ^^, it will remain so in some interval t^ < t < t* until at t = t*, \F{t) — Smx\ > iim^g. Again at t = t^+i, dx/dt may be equal to V. If so, t = ti+i is a free boundary. If it is assumed that mass does not j u m p and there is no abrupt change in the velocity of the mass, then it can be assumed t h a t x{t) and dx/dt are continuous functions of time and dx/dt — 1/ at the free boundary. These assumptions provide boundary conditions at each free boundary. There is a fundamental difference between this problem and the free boundary Problems 1.1.1. to 1.1.6. In the earlier problems, free boundaries arose due to the physical situations or the mathematical nature of the solution such as a weak solution, but in the present problem free boundaries arise due to discontinuities in the physical properties of the solution.
A Mass
Spring
AI—frnm A
B
> F(t)
Conveyer belt
Fig. 1.1.3. Frictional oscillator P r o b l e m 1.1.8.
I m p a c t of a V i s c o - P l a s t i c B a r o n a R i g i d Wall
A bar of length b made up of visco-plastic incompressible material moving with constant velocity —VQ hits a rigid wall at time ^ = 0. We consider a one-dimensional problem in which X-axis is taken opposite to the initial motion of the bar and the rigid wall is taken at x = 0. After the bar hits the wall, compressional stresses develop in the bar giving rise to visco-plastic flow of the material in the region. If V(x, t) is the velocity of the bar for ^ > 0, then the gradient of V{x, t) or the velocity of deformation can be expressed as follows (cf. [16]). ^
dx
, \r\>
To, To > 0, r < 0, (1.1.59)
0 . , |r| < To.
Here, r represents compressional stress, which is negative as the x-axis is oriented opposite to the direction of motion of the bar, TQ is the stress at the limit point and //, is the coefficient of viscosity of the material.
12
The Stefan Problem and its Classical Formulation
It will be assumed that any disturbance is propagated over the whole bar instantaneously. The equation of motion in the visco-plastic region can be easily obtained by using Newton's second law of motion. We have dV dr ^"dt^'dx^
d^V '^^'
0 < X < So{t), 0 < ^ < t„ 5o(0) = 0,
(1.1.60)
where p is the density of the material and x = So{t) is the interface between elastic and visco-plastic regions. In the elastic region dV — = 0, \T\
(1.1.61)
Integrating (1.1.61), we obtain V = -X{t),
X(0) = yo-
(1.1.62)
In obtaining (1.1.62) it has been assumed that the elastic portion of the rod moves like a rigid body. Using again Newton's second law of motion at x = SQ, we obtain - M ^
= T\S„+FO,
(1.1.63)
where M is the mass of the elastic part and FQ is the area of cross section of the rod assumed to be uniform. Since M = Fop(b — So(t)), equation (1.1.63) becomes
dt
p(b-So{t)y
^ • • ^
In obtaining (1.1.64) it has been assumed that stress is continuous at the interface x = So{t). Using the condition that V{x,t) is also continuous at x = So{t), we get V(So,t) = -X{t).
(1.1.65)
Also ^V{So.t)
= 0.
(1.1.66)
The initial and boundary conditions for V(x, t) are V(x, 0) = -Vo, 0 < X < 6; ^(0, t) = 0, t > 0.
(1.1.67)
The functions V{x,t), So{t) and X{t) are to be determined. An approximate solution of the above problem has been discussed in [16]. The free boundary in this problem arises due to the change in the physical properties of the system.
1.1. Some Stefan and Stefan-like Problems Problem 1.1.9.
13
A Problem with Discontinuous Moving Boundary
Discontinuity of the free boundary in multi-dimensional problems can be easily demonstrated geometrically For example, a piece of ice floating in water breaks into two pieces after melting for some time. A discontinuous moving boundary in a one-dimensional problem does not commonly occur. In [17] an interesting problem of discontinuous moving boundary which is associated with the diffusion of moisture in a porous capillary tube of length unity is discussed. At time t = 0, the portion of the tube 0 < x < XQ, XQ < 1 is filled with moisture and the remaining portion is dry. The temperature T(x, t) of the moisture is less than the boihng temperature T = 0, except at x = XQ where T = 0. The temperature of the dry air in some neighbourhood of x = XQ on the right is greater than zero and in the remaining portion, the temperature of the air is less than zero. There is a continuous flow of moisture into the tube at x = 0. At x = 1, the dry air is getting heated, causing evaporation to take place and making the moisture advance into the dry air. Let Ti(x, t) and T2(x, t) be the temperatures of the moisture and the dry air respectively, Wi{x,t) the concentration of the moisture, and x = S{t), the equation of the free boundary which is the interface between moisture and the dry air. Under certain initial conditions it may happen that at some instant of time, say t = m, m > 0, the temperature in some neighbourhood Sm of the moving boundary becomes less than or equal to zero. In this case the moisture will advance with a jump into the dry part, i.e., S(m+) - S{m-)
^6m>0.
(1.1.68)
The problem is concerned with finding Ti, T2, Wi and S{t). The formulation of this problem is as follows: Differential
Equations
dT d^T ^ = « ? ^ . (a^,<)eA, z = l,2,
~^
=d-^,
(x.^)eA,
L>i = {(x, t) : 0 < X < S{t), 0 < ^ < m} U {(x, ^) : 0 < X < S{t), m
0
(1.1.69)
(1.1.70) t*},
{(x, t) : S{t) < x < 1, m < t < t^}.
Here a^ is the thermal diffusivity and d is the mass diffusivity Equation (1.1.69) is the heat conduction equation and (1.1.70), the mass diffusion equation (Pick's law). Initial conditions Ti(x,0) - 01 (x) < 0, Wi{x,0) = '0i(x) > 0; 0 < X < xo,
(1.1.71)
T2(x, 0) = 02(a:), Xo < X < 1 ; (/)2(xo) > 0 and 02(1) > 0.
(1.1.72)
14
The Stefan Problem and its Classical Formulation Boundary conditions at x = 0 and x = I Ti(0,t) = /i(t) < 0, Wi(0,t) = gi{t) > 0; 0
0 .
(1.1.73) (1.1.74)
Boundary conditions at the moving boundary x = S{t), t ^ m {Ti{x,t),
dx
Wi(x,t)},^sw
= 0,t>0,
5(0)=a;o,
= a{T2{S{t)+0,t)~Ti{S{t)~0,t)),
t>0.
(1.1.75)
(1.1.76)
=S{t)
Here, K is the thermal conductivity and a is the heat transfer coefficient. The convective boundary condition in (1.1.76) arises because of the discontinuity of temperatures at x = S{t), t ^ m. Heat balance at x = S{t) implies C2P2T2(S{t)+Q,t)'^+
K2^{S{t)+Q,t)
-
K,'^{S{t)-0,t) = -dq, — iS{t)-0,t).
(1.1.77)
Here, C is the specific heat, p the density and qi the latent heat of evaporation. Definitions of different parameters have been given later in § 2.1.3. The derivation of (1.1.77) is based on the law of conservation of energy (see § 1.4.7) A sufficiently small real number e > 0 exists such that T2{x, t) > 0 for S{t) <x<
S{t) + £, t^m.
(1.1.78)
For t = m, Ti(x,m+) = T 2 ( x , m - ) , and Wi{x,m+)
= 0;S{m-)
<x < 5'(m+), (1.1.79)
m± = m ± 0, T2(x,m) < 0, S{m-)
<x<
S(m-{-).
(1.1.80)
The existence and uniqueness of this problem under suitable assumptions have been discussed in [17] and sufficient conditions for the existence of a discontinuous moving boundary are given in [18]. Discontinuity in the temperature and in the free boundary is a typical feature of this problem. Problem 1.1.10.
Penetration of Solvents in Polymers
Consider a slab of a glassy polymer, such as methyl methacrylate in contact with a solvent, n-alkyl alcohol [19]. If the solvent concentration exceeds a threshold value, say.
1.1. Some Stefan and Stefan-like Problems
15
9 > 0, then the solvent moves into the polymer, creating a swollen layer in which the solvent diffuses according to Pick's law for mass diffusion. If W(x, t) is the concentration of the solvent in the polymer and x = S{t) is the free boundary representing the penetration depth of the solvent in the polymer, then W and S satisfy the following equations. Wt - W^xx = 0, {x,t) G A . = {{x,t) :0<x< W{0,t) = l, S{t) = f{W{S{t))},
S(t), 0
0
W^{S{t), t) - -S{t)[W{S{t),
t) + q], 0
t,},
(1.1.81) (1.1.82) (1.1.83) (1.1.84)
In these equations, normalized solvent concentration is represented by W + q. W represents the excess concentration, normahzed to 1 at x = 0. Equation (1.1.83) describes the prescribed penetration law and (1.1.84) arises from the mass conservation at the free boundary. The well-posedness, existence, uniqueness, etc., of the solution of the above problem have been shown in [19]. A numerical method for its solution has also been suggested in [19]. Some more mathematical models describing the crystallization of polymers and their mathematical analysis are presented in [20]. In (1.1.83), S{t) is a function of the concentration. Problem 1.1.11.
Filtration of Water Through Oil in a Porous Medium
Consider a one-dimensional problem in a semi-infinite porous medium x > 0 of porosity m. At time ^ = 0, the region 0 < x < b is filled with oil and the region 6 < x < oo is filled with water. Water percolates into oil, and so for i > 0 there will be three regions. Water-filled region, b < x < oo, will have hundred percent saturation of water; oilbearing region, 0 < x < S{t), will have hundred percent saturation of oil; in the region S{t) < X < 6, both water and oil mixture will be present. This region can be called an intermediate zone. S{t) is the free boundary. The oil content of this intermediate zone changes with time, and tends to reach a limiting state called the residual oil saturation state. This stage is characterized by the fact that if time is counted from the moment of passage of the oil-water contact boundary past a fixed element of volume, the degree of oil saturation of this volume will not depend on time and the flow in this region will be of one phase. It will be assumed that water filters through the intermediate zone while the oil in this zone remains stationary [21]. However the rate of penetration of water in the intermediate zone is lower than in the water-filled zone as the oil concentration in the intermediate zone is greater than zero. Under appropriate assumptions, the following one-dimensional model is obtained.
16
The Stefan Problem and its Classical Formulation
-. = - ^ 1 ^ ; a^f^ = % , 0<x<5(tM>0. fi2 OX
ox^
(1.1.87)
at
The subscripts 0,1,2 stand for the water-fihed, intermediate and oil-rich zones, respectively, u^ and PJ, i — 0,1,2, denote the filtration velocities and pressures in the three different zones. The constants A,,a^ and //,, i = 0,1,2, denote the coefficients of permeability, piezoconductivity and viscosity of the zth zone. The equation of velocity in terms of pressure gradient is the result of Darcy's law [8] and the pressure obeys the equation of piezoconductivity. Boundary conditions at x = 6 and x = S{t) follow from the continuity of velocities and pressures and are given by uo = ui, po = pi] at X = 5, ui = U2, P\^P2\
d^tx = S(t).
(1.1.88) (1.1.89)
At the free boundary, an additional condition is required which is obtained from the 'mass balance' consideration m{\ - 6i - 82)—= ui.
(1.1.90)
Here, 61 and 82 are the contents of water and the residual oil saturation, respectively in the transition zone, and m is the porosity of the stratum. To complete the formulation of the problem, suitable initial and boundary conditions for pressure should be prescribed. For example in [21], the initial pressure in the entire stratum is assumed to be constant. Similarly it is assumed that pressure has a constant value at the boundary x = 0. The zero reference point for calculation of pressure is so chosen that p2[Q,t) = 0. With the above assumptions the initial and boundary conditions can be written as Pi\t=o = P = constant, z = 0,1, 2; p2\x=o = 0.
(1.1.91)
An approximate solution under the quasi-steady approximation, valid at the initial stages has been obtained in [21] with constant parameter values . The boundary conditions (1.1.89) do not contain the velocity of the free boundary. Problem 1.1.12.
Obstacle Problem for a String
This problem belongs to a class of problems which have a variational inequality formulation. Three different types of formulations of this obstacle problem are possible and their equivalence is discussed in Chapter 7. A new notion of codimensionahty-two of the free boundary is associated with this problem and this wih be discussed briefly in § 1.2. Consider a weightless elastic string which is held tight between two fixed points Pi = (0, 0) and P2 = (6, 0), 6 > 0 in P^-plane. This string is displaced upwards by a rigid body called an obstacle (cf. [22, 15]). A view of the cross section of this system of string and obstacle from above in P^-plane is shown in Fig. 1.1.4. Let y = ip(x) be the equation of the cross section of the obstacle in P^, i.e., the equation of the curve Q1ABQ2. '0(x)
1.1. Some Stefan and Stefan-like Problems
17
is assumed to be sufficiently smooth. We shall relax this smoothness condition later (see § 7.2.5). Let y = u{x) be the equation of the string in the equilibrium position or the equation of the curve P1ABP2. The problem is to find the function u{x) and the arc AB of the string which is in contact with the obstacle and on which u{x) = ip{x). The points A and B are free boundaries and in the present case we can call them 'free points'. Once A and B are determined, the arc AB is also determined because ip{x) is known. The formulation of the problem is as follows: u{0) = u{b) = 0,
(fixed end conditions),
(1.1.92)
u{x) = ip{x) on AB; u{x) > ip{x) on P1ABP2,
(1.1.93)
u'{x) < 0,
(1.1.94)
u{x) > ip{x) =^ u"{x) = 0.
(1.1.95)
The second equation in (1.1.93) implies that the string does not penetrate the obstacle. The concavity of the string as viewed from the x axis imphes (1.1.94) and the tightness of the elastic string implies that the portions Pi A and BP2 are straight lines and so u"{x) = 0. The two boundary conditions at the free boundary can be obtained from the continuity of u and du/dx^ i.e.. [U]A
0,
= [U]B •
(1.1.96)
where [/] stands for the jump in the quantity under consideration at the given point. Equation (1.1.95) is equivalent to the following condition [u{x)-^{x)]u"{x)
= 0.
(1.1.97)
In view of (1.1.93)-(1.1.95), equation (1.1.97) is valid. When u{x) = ip{x), (1.1.97) is satisfied and when u{x) 7^ ip{x) then the second equation in (1.1.93) implies u{x) > il){x) and from (1.1.95), u"{x) = 0 and therefore (1.1.97) is satisfied. If (1.1.97) holds and if u{x) > ipix), then u"{x) = 0, which is (1.1.95). The formulation (1.1.92)-(1.1.95) is equivalent to the formulation given by (1.1.92)—(1.1.94) and (1.1.97). Let us call the formulation (1.1.92)—(1.1.95) as Problem {Ri) and the formulation equivalent to Problem (Ri) as Problem (i?2)- Consider the following minimization problem which is concerned with the minimization of the energy of the above string. J 2
min / u>^ PiJ
-^]dx dx j
, Vw(x) eV,
(1.1.98)
where V = {u : u{0) = u(b) — 0, u and dujdx are continuous, and u satisfies relations (1.1.94) and (1.1.95)}.
18
The Stefan Problem and its Classical Formulation
We shall call this formulation in (1.1.98) as Problem {Rz)- This is a fixed domain formulation as it does not consider separate formulations on different portions of P1ABP2. In view of the equivalence of Problems {R2) and {R3) which will be established in Chapter 7, the formulation {R2) is also called 'variational formulation' of the obstacle problem. A variational inequality formulation of this problem has been given in Chapter 7.
y=vKW A/7 / //N
///////// /////////// /////////// '//////////// ////////////
Po=(b,0)
p,=(ao) Fig. 1.1.4. Obstacle and the stretching of the string.
1.2
Free Boundary Problems with Free Boundaries of Codimension-two
In Problem 1.1.12., the free boundary consists of just two points A and B. Therefore the dimension of the free boundary is zero, whereas the spacial dimension of the problem is two. Another example of a free boundary of codimensional-two (or free points) is the tip of a propagating line crack in an elastic medium. We shall see later in § 3.2.2 that in the problems concerning initiation of solidification/melting along a known surface, codimension-two free boundaries could be time dependent. If the spacial dimension of the problem is n (the problem could be transient) and the dimension of the free boundary is n — 2, then we say that the free boundary is of codimension-two. Several problems with free boundaries of codimension-two have been discussed in [23]. In Problems (1.1.1)(1.1.11), the dimension of the free boundary in only one less than the spacial dimension of the problem. The distinction between the free boundaries of codimension-two and the free boundaries of codimension-one is not superficial but it raises some serious questions [23], some of which are mentioned below. 1. Can these problems with free boundaries of codimension-two, in some sense such as hnearization, be modelled as limits of free boundary problems of codimension-one? 2. What can be said about the existence, uniqueness and regularity of solutions to models as stated ? In particular, is any of the information redundant ?
1.3. The Classical Stefan Problem in One-dimension and the Neumann Solution
19
3. What methodology is available to solve these problems explicitly? No systematic methodology has yet been proposed for nonlinear problems of this type. 4. If solutions exist, are they stable to perturbations in the direction parallel to the free curve? 5. Is there a possibility of generating the models either to make their analysis or numerical solutions easier?
1.3
The Classical Stefan Problem in One-dimension and t h e N e u m a n n Solution
Stefan problems are free boundary problems with some special features which can be easily explained with the help of the following one-dimensional solidification problem. At time t = 0, a pool of liquid at constant temperature To > T^, occupies a semi-infinite region x > 0. Tm is the temperature at constant pressure at which the liquid solidifies. It is called freezing temperature or phase-change temperature. In the case of pure metals such as copper, Tm is called melting temperature also because in the case of pure metals freezing and melting temperatures are the same. Tm is also called equilibrium phasechange temperature as both sohd and liquid phases can stay together in thermodynamic equilibrium dX T = Tm (cf. § 2.1.4.). The fixed boundary a: = 0 is being cooled, for example, temperature T^ < Tm is prescribed at a; = 0. In this case soHdification starts instantaneously at x = 0. For any ^ > 0, the region 0 < x < oo will consist of sohd and hquid phases with the solid phase occupying the region 0 < x < S(t) and the liquid phase the region S{t) < x < oo. S[t) is the free boundary or the phase-change boundary/interface. This solidification problem can be formulated mathematically as follows: In the solid region f)T fP'T CsPs^ = KS-QJ:, Ts{x,t)\,^o
0 < X < 5(<), t > 0,
= Tt
(1.3.1)
t>0.
(1.3.2)
In the liquid region CLPL^
+ CiPLU,^
= K^-^,
<S{t)<x<<x>,
TL{x,t)\t^o = To, ri.(x,0|.^oc = To.
t>0,
(1.3.3) (1.3.4)
20
The Stefan Problem and its Classical Formulation At x = S{t)
K s ^
- KL^
TS = TL = Tm.
(1.3.5)
= {Ips + {PLCL - PsCs)Tm}^,
(1.3.6)
S{0) = 0.
(1.3.7)
Equations (1.3.1)—(1.3.6) are not dimensionless. Temperatures Tf,, To, Tm and thermophysical parameters p, C, K, and / are known quantities. The subscripts S and L stand for solid and liquid regions. The symbols used in (1.3.1)—(1.3.6) are explained in the 'List of Notations'. The definitions of thermophysical parameters C, K, /, etc., will be discussed later in § 2.1.3. At present, our interest is to highlight some basic features of Stefan problems. Equation (1.3.1) is the Fourier heat conduction equation derived on the basis of Fourier's law given in (1.3.8) for heat conduction in a homogeneous isotropic medium. q=-K
grad T.
(1.3.8)
Here, qis called a heat flux vector and gives the heat flow at a spacial point at any instant of time t- q-n gives heat flow per unit area, per unit time in the direction of the outward drawn unit normal n to the surface of the region under consideration. The derivation of heat conduction equation is given in many books such as [24] and is obtained on the basis of principle of conservation of energy (cf. § 1.4.7). The heat equation (1.3.1) is based on a macroscopic model but it can also be derived on the basis of a microscopic model. The transport of heat energy takes place due to the random motion of molecules. The parabolic heat equation as well as hyperbolic heat equations can also be derived by considering discrete random walks of molecules [25]. If the densities of solid and liquid phases are not equal, then liquid may acquire motion whose velocity u^ in the one-dimensional case can be obtained from the mass balance condition at the interface given below. pLUa: = {pL-ps)-^-
(1-3.9)
If ps > PL, then there will be a shrinkage of total volume in solidification, and u^j, will be negative as dS/dt is positive in the above problem. If ps < PL-, then there will be an expansion of the volume and u^ will be positive as dS/dt is positive. Equation (1.3.3) is the Fourier's heat conduction equation for the liquid in motion. Boundary conditions at the fixed boundary x = 0 could differ from that in (1.3.2) (see § 1.4.4.) but in the 'Neumann solution', a constant temperature is prescribed at x = 0. At t = 0 it was only the liquid phase therefore the initial condition has been prescribed only for the liquid. In the present problem To is a constant. If Tm is a constant, then the conditions in (1.3.5)
1.3. The Classical Stefan Problem in One-dimension and the Neumann Solution
21
are called isotherm conditions. Equation (1.3.6) is the dynamic energy balance condition. Both (1.3.5) and (1.3.6) are based on the assumption of instantaneous achievement of local thermodynamic equilibrium at the phase-change interface. —KidTi/dx is the heat flux from the liquid outwards (the outward normal to liquid at S{t) points into the solid) and —KsdTs/dx is the heat flux from the solid outwards (the outward normal to solid at S{t) points into the liquid). Equation (1.3.6) can be derived by using the energy conservation law. It gives the energy balance at the phase-change boundary. In § 1.4.1, the derivation of the equation (1.3.6) has been obtained in a more general context. When the phase-change takes place from liquid to solid, the latent heat / released by the system is removed by conduction. Equation (1.3.6) is a dynamic compatibility condition and is commonly known as the Stefan condition. It is called a condition of Rankine-Hugoniot type. Equation (1.3.6) can also be stated in the form
OX
[H]^^. liquid
(1.3.10)
liquid
Here, H is the enthalpy per unit volume, which is the sum of the latent heat plus the sensible heat, and Vn is the normal component of the velocity of the interface with the unit normal n pointing into the liquid. Melting Problem The problem discussed in (1.3.1)—(1.3.6) is a solidification problem as the phase changes from hquid to sohd. The melting problem in which the phase changes from sohd to liquid can be easily formulated by making appropriate changes in (1.3.1)—(1.3.6). For example, (1.3.3) is still valid for liquid but liquid region is now 0 < x < S{t). Similarly other equations can be modified. During solidification, the system releases latent heat but during melting the system 'absorbs' latent heat. Since we take / > 0, a minus sign should be put for heat absorbed. Equation (1.3.10) is stiU valid for the problem of melting provided the changed directions of normals to the two regions at the free boundary are taken into account. Since the formulation of both solidification and melting problems has the same mathematical structure, they are said to be mathematically analogous. If a solution to one of them is obtained, then the solution to the other problem can be obtained by making some minor changes in it. It is for this reason that it suffices to study either of them. Neumann Solution The method of analytical solution of (1.3.1)—(1.3.7) which is discussed below is based on some special type of solutions of heat equations (1.3.1) and (1.3.3), which can be written in terms of error and complementary error functions [26]. An exact analytical solution of the Stefan problem even in the one-dimensional case is possible only in few simple cases. If in (1.3.2), To is time dependent or if a constant flux is prescribed at x = 0 then the method discussed below fails to provide a solution. The solution of (1.3.1) can be written
22
The Stefan Problem and its Classical Formulation
'""^"'my^^'''-
<""i
A and B are constants to be determined. It is easy to see that Ts satisfies (1.3.1) and TL satisfies (1.3.3) with u^ given in (1.3.9) and the initial condition given by the first condition in (1.3.4). Let S{t) be given by S{t) = 26{ksty^,
6 an unknown constant.
(1.3.13)
On a: = S{t) (given in (1.3.13)), both Ts and Ti are constant, and S'(O) = 0. Now we have three unknowns, namely, A, B and 6, and three conditions, i.e., (1.3.6) and isotherm conditions for Ts and TL. After some algebraic manipulations, the unknown constants can be obtained thus. A^{T^-T,)lerm.
(1-3-14)
B = {To- Trn)/eTic{Spskf/{pLkf)), e-'' erf(5
(To - Tm)KL k'J'e-^''pl ^s/plk,) Trr^Ks kferic
{{Sps kf/{PL
l = l + {CLPL/ps-Cs)Tm.
^f))
(1.3.15) ^[^1/2 ^sT^'
(1.3.16)
(1-3.17)
This completes the solution of (1.3.1)—(1.3.7). When ps = PL = P, this solution is called Neumann solution which was given by Pranz Neumann [27] in 1860. The first pubhshed discussion of such problems seems to be that by Stefan [28] in a study of the thickness of polar ice. The problem discussed above is called a two-phase Stefan problem as there are two distinct phases. If the liquid is initially at the equihbrium temperature T^, then heat conduction will take place only in the solid phase. In principle two phases will be present, but since there is no temperature gradient in the liquid, such problems are called onephase Stefan problems. If there are n (> 2) distinct phases separated by (n — 1) distinct phase-change boundaries, then we have an n-phase Stefan problem. It is not possible to give a precise definition of a Stefan problem but in the light of the above discussion some of the characteristic features of Stefan problems can be described as follows: (1) the transport phenomenon is that of heat transfer governed by parabolic equations, (2) any two distinct phases are separated by a sharp phase-change interface, (3) at the
1.4 Classical Formulation of Multi-dimensional Stefan Problems
23
phase-change boundary, temperature is prescribed which is known, (4) heat flux is discontinuous across the phase-change boundary and the latent heat is released or absorbed at the interface. Phase changes in which the heat flux is discontinuous are known as first-order phase transitions. We shall learn more and more about these characteristic features of Stefan problems as we proceed further. In many physical problems although a free boundary exists, there is some deviation of features in them from the characteristic features of the Stefan problem. It has become almost customary to indicate such problems by a suitable nomenclature. For example, if a hyperbolic equation is considered instead of a parabolic equation, then it is called 'hyperbolic Stefan problem'. Similarly, we have a 'Stefan problem with kinetic condition' if a kinetic condition is introduced at the free boundary or 'Stefan problem with supercoohng' if the liquid is supercooled. The term 'classical' is generally used to distinguish the classical formulation from the weak formulation (§ 5.2 for the weak formulation of the classical Stefan problem). In a classical formulation it is assumed that there exists a suflfiiciently regular sharp interface, which separates two distinct phases. Two phases will be considered different if they differ in their composition, structure, or properties. At the phase-change interface, two boundary conditions are prescribed, one in the form of temperature and the other arising from the energy balance at the interface.
1.4 1.4.1
Classical Formulation of Multi-dimensional Stefan Problems Two-phase Stefan problem in multipledimensions
We shall now extend the mathematical formulation given in (1.3.1)—(1.3.7) to multidimensional problems. For simplicity we consider R^. Consider a heat-conducting openbounded region G C R^ whose boundary dG is smooth, for example dG G C^^^, for some T] > 0. Let G{t) = G x {t} and dG{t) = dG x {t}, 0 < t < t^ < oo ioi some suitable real number t*. G^* = Do
(1.4.1)
S{t) = {(x, t) e G'* I $(a:, ^ = 0, a: G G and ^ G [0, t,]},
(1.4.2)
where
$ G C\G^*) and V:,^{x,t) ^^ 0 on S{t). ^{x,t) phase 2. (5 is the closure of G.
< 0 is the phase 1 and ^[x,t) > 0 is the
We assume that G^* also admits disjoint decomposition G'* - Gl* U r(t) U G*2%
The Stefan Problem and its Classical Formulation
24
where G'{ = Uo
E
^ d d' ^'Jk{^:t) dxjdxi ^Y.b'j{x,t)^-+r\x,t), dxj
2 = 1,2.
(1.4.3)
Here, a'-j^, V^ftj/e, ^lo'^'jk^ ^j^ ^ x ^ j , and r^ are continuous in G^* ;x e G and x = (xi, 2:2, Xs) Consider the following solidification problem in G{t). Conservation of energy m Gi(t) QA
dT, dt
V^T^, {x,t) e G,{t), z = 1,2, 0 < ^ < t*.
Fig. 1.4.1. Solid and liquid regions at a fixed time in a 2-D problem. Geometry number 1.
(1.4.4)
Fig. 1.4.2. Sohd and hquid regions at a fixed time in a 2-D problem. Geometry number 2.
Boundary conditions on dGi{t) T,{x, t) = g^{x, t), X e ^G^(t), 2 = 1,2; 0 < t < t*, Qi < Tm and .92 > ^m-
(1.4.5)
1.4 Classical Formulation of Multi-dimensional Stefan Problems
Fig. 1.4.3. Cylinder over G(0) in Fig. 1.4.1.
25
Fig. 1.4.4. Cylinder over G{0) in Fig. 1.4.2.
Initial conditions T,(x,0) = /,(x), X e G^{0), 2 = 1, 2,
(1.4.6)
/i < Tm and /2 > Tm. Isotherm conditions on <^(x, t) = 0 T^ = Tm, z = l,2.
(1.4.7)
Energy balance at ^{x,t) = 0 [ -q]l • n = pjv
• n,
(1.4.8)
where q = (Q'I, 92, ^s) is the heat flux vector for anisotropic bodies and is defined as 3
QJ^
^ ^ - - E ^ ^ . dxj' ^ ^ z = 1,2,3.
(1.4.9)
j=i
Here, Kij are called thermal conductivity coefficients, n is the unit outward normal to the solid phase at S{t), and V is the velocity of the free boundary. Equation (1.4.8) is a generalization of (1.3.10) to a multi-dimensional case. If the heat equation (1.4.3) is written for anisotropic bodies, then a'j,^{x,t) are known functions which can be identified with Kij{x,t) in (1.4.9). In a more general case K^j could be functions of temperature also and in that case the operator Vi will change (see (1.4.29)). The coefficients h]^[x,t) could be the space derivatives of K^j{x,t) and/or convective terms arising due to the motion of the zth phase. In the case of motion of phases, total derivative of T, with respect to time should be considered. If so, DT
Dt
dT
^
BT
(1.4.10)
26
The Stefan Problem and its Classical Formulation
where li^, j = 1, 2, 3 are components of the velocity of the ith. phase. Motion in the hquid phase may arise due to natural convection and/or shrinkage or expansion of the volume, etc. In (1.4.4), b^ji^{x, t) could be taken to be either known or unknown quantities, r' is the rate at which heat is generated/annihilated per unit volume. If ^^^(x, t) are unknown and arise due to the liquid motion, then (1.4.4)—(1.4.8) have to be supplemented with other equations to determine them. The motion of the solid phase is generally neglected but the motion of the melt plays an important role in solidification/melting problems. In the onedimensional case, Ux could be easily determined with the help of mass balance condition (1.3.9). In the multi-dimensional case, the velocity of the melt can be determined only by supplementing (1.4.4)—(1.4.8) with the equations of fluid dynamics such as Navier-Stokes equations discussed in § 1.4.7. Instead of temperature-prescribed boundary conditions on dGi{t), i = 1,2, other types of boundary conditions can also be prescribed provided solidification takes place in such a way that only one phase-change boundary separates the two phases. This restriction is inherent in the classical formulation. Solidification/melting problems could be much more complicated. For example, if ice pieces are put in a glass of water, then a free boundary exists at the boundary of each ice piece. Two free boundaries which may be disjoint at some instant of time may join at a later instant. A piece of ice on melting, may break into two. Such problems are extremely difficult to investigate. In (1.4.7) it has been assumed that there is thermodynamic equilibrium at the phase-change boundary. Equation (1.4.8) can be expressed as
E :^'^^k^T^ « 7"f^c^7dx^ ^ ^ dx, = -pi "^'= dx, dxk- E^-1^1 " -^''
df
(1-4.11)
Equation (1.4.11) can be obtained as follows. Let q' be the heat flux vector in the ith phase given by (1.4.9) with Kjk replaced by Uji^. Then r-n
= q' • grad$/|grad$| = U— + q\— + (?^—j / |grad$|, i = 1,2, (1.4.12)
we have , d^ d^dx d^dy d^ dz ^ d^, ^(x,,,.,*) = ^ + ^ ^ + ^ ^ + ^ ^ = 0 ,
/. . . .x (1.4.14)
{V-n)\gmd^\ = - — .
(1.4.15)
Substitution of (1.4.13) and (1.4.15) into (1.4.8) results in equation (1.4.11). The negative sign on the r.h.s. of (1.4.11) will arise in (1.3.6) also if we take ^{x,t) = x — S{t) in (1.3.6).
1.4 Classical Formulation of Multi-dimensional Stefan Problems
27
The energy balance condition (1.4.8) has been obtained on the basis of the principle of conservation of energy stated in § 1.4.7. The detailed derivation of (1.4.8) using energy conservation principle is given below. Consider the three-dimensional problem of solidification of a melt in which we take densities of solid and liquid phases to be the same and consider an isotropic case for the sake of simplicity. Let P be a point on the interface S{t) at some given time t, and Q be a point on S{t + A^). Q hes along the vector normal to S{t) at P as shown in Fig. 1.4.5. Solidification progresses from P to Q. Take PQ to be the direction of the vector Hx normal to the surface S{t) at a fixed time where fix = (71^1,723.2,72x3), n^^ — cos(n, x^), i = 1,2,3, n is the unit vector normal to the interface S{t) and n = (nx.^t)- ^t = cos(72,^), t is the unit vector in the direction of t and x^, z — 1,2,3 are unit vectors in the xi, X2, X3 directions, respectively. Here, xi, ^2, and 0:3 are Cartesian coordinates of a point in B?. Let Alt; be a small area on S{t) enclosing the point P. Construct a cylinder with base Alt;, height \PQ\ and axis PQ. The latent heat released during the solidification of this volume element is pl^w\PQ\. The rate at which latent heat is released during solidification of this volume element is PQ lim p/l——lAti; = plV • fix Aw.
(1.4.16)
S(t+At)
nx
Fig. 1.4.5. Geometry in the derivation of the Stefan condition Here, V is the velocity of the interface. On using the energy conservation principle in the cylinder of volume Aw\PQ\ which requires calculating the difference in the rates at which heat flows into the cylinder across the left face and leaves through the face on the right and adding to it the rate at which heat is released and cancelling Aa;, we obtain
on I
an
where fii = —n and ni is the outward normal to the base of the cylinder at P.
(1.4.17)
28
The Stefan Problem and its Classical Formulation
1.4.2
Alternative forms of t h e Stefan Condition
The Stefan condition (1.4.11) in multipledimensions can be expressed alternatively in a form more suitable for analytical and numerical treatment. It will be assumed that $(x, y, z,t) = 0 can be written as ^(x,y,z,t) = z- S{x,y,t)
= 0.
(1.4.18)
Alternative expressions Hke x — f{y, z, t) or y—g{x, z, t) can also be used for . Sometimes the physics of the problem requires the free boundary to be expressed in the form t = w{x,y,z) (see (3.3.46)). Differentiation of (1.4.7) with respect to x and y gives dT\__dT]d^ dT]__dT\d^ dx dz dx ' dy dz dy' Prom (1.4.18), we have d^ dx
d^ dz _ d^ dz dx ' dy
d^ dz dz dy
d^ ' dz
Equation (1.4.11) can be rewritten as
On using (1.4.19) and (1.4.20) in equation (1.4.21) the latter can be written as
Note that in (1.4.22), the derivative of temperature is with respect to z but not with respect to x and y. Equation (1.4.22) looks like the Stefan condition in a one-dimensional problem. In an isotropic cetse, a^jj^ = KiSjk, where Sjk is the'Kronecker delta function'.
(1.4.23)
Using (1.4.23) in (1.4.22), the Stefan condition can be expressed as
fdsy
[dsyli^dTi
r. 9T2]
^os
,. . . . x
On using (1.4.19) and (1.4.20), we have (1.4.25)
iv$i
ivT,r
Also on the free boundary V-n = V-VT^\\/T^\ = -dT,dt/\VT^\,
z = l,2.
(1.4.26)
1.4 Classical Formulation of Multi-dimensional Stefan Problems
29
Isotherm conditions (1-4.7) can be differentiated with respect to t to obtain the second relation in (1.4.26). Substituting (1.4.26) in (1.4.8), we obtain \K,VT,\ - |i^2VT2| = -pi-^/\VT,\ = -pi^/lVnl
1.4.3
(1.4.27)
The Kirchhoff's transformation
Consider the case in which the thermal conductivity varies with the temperature, but is independent of position and time. The heat equation for isotropic material in this case can be written as dT = div{K(T)gmdT) + A{T,x,y,z,t), pC—
A is the rate at which heat is supplied per unit volume. Kirchhoff 's transformation is defined as [24], 1 ^ 0 = —- f K{a)da, where i^o is the value of K{T) when T = 0. It can be seen that dO _ K dT 86 _ K_dT_ d^ _ K dT_ 86 _ K dT KQ dt ' dx KQ dx ' dy KQ dy ' dz KQ dz ' dt and
(1.4.29)
(1.4.30)
where K and C are now functions of 6. In many cases, for example, in metals near absolute zero temperature, both K and C are proportional to the temperature and pC/K can be taken to be a constant. If A is independent of the temperature, then there will be no difference between the structure of the Fourier's heat conduction equation and equation (1.4.31). However if we use Kirchhoff's transformation (1.4.29), the choice of boundary conditions will be hmited. When T is prescribed as T == f{x,y, z,t) on the boundary, the expression f{x,y,z,t)
(9 = —
/" K{a)da,
(1.4.32)
0
gives the boundary condition for 6. Similarly, if the normal derivative of temperature T is prescribed, the normal derivative of 6 can also be prescribed. The extension of the formulation (1.4.4)—(1.4.8) to R^, n > 3 is simple. Many generalizations of this formulation are possible and some of them will be discussed later.
30
The Stefan Problem and its Classical Formulation
1.4.4
Boundary conditions at the fixed boundary
(A) Standard boundary conditions (AI) Type I boundary condition In the boundary condition of this type temperature is prescribed. Let dG be the boundary of the region G C R^. Type I boundary condition is of the form T\dG = /(x, y^z,t),t>
0, (x, y, z) e dG.
(1.4.33)
If / < Tm{> Tm) then sohdification (melting) starts instantaneously i.e., there is no waiting time for the phase-change to take place. Since energy can neither be produced nor disappear at infinity if G is unbounded, we have lim T ( x , t ) - T ( x , 0 ) , t > 0.
(1.4.34)
|x|—>oo
In order to obtain a physically meaningful solution to the diffusive heat equation for unbounded domains, it is required that dT lim ^r-(x,t)
- 0 , t>0.
(1.4.35)
Temperature-prescribed boundary conditions are known as Dirichlet type boundary conditions. Sometimes, while constructing approximate solutions more than the prescribed number of boundary conditions are required to match the number of unknowns. In such a case additional boundary conditions are generated. Let us consider a radially symmetric one-dimensional heat conduction problem in which the temperature at r — a is prescribed as T{r,t)\r^a = f{t).
(1.4.36)
If (1.4.36) is differentiated with respect to time and the Fourier's heat equation is used, then
dT 'dt
=^ ^
+-^1
=/'W
(1-4.37)
r=a
Equation (1.4.37) provides one more boundary condition for the problem. (All)
Boundary condition of radiative-convective type
This boundary condition can be written as
dn
= 0E{T' - n)\aG + [CxT - C2To)|aG.
(1.4.38)
1.4 Classical Formulation of Multi-dimensional Stefan Problems
31
Here, n stands for the unit outward normal, /3 is Stefan-Boltzmann constant, E is the emissivity of the surface which is defined as the ratio of the heat emitted by the body to the black body at the same temperature, To is the ambient temperature, and Ci and C2 are functions of time, p is positive and has dimensions [watt]/[meter]^x [Temperature]"^, 0 < E < 1. From this boundary condition, three types of well known boundary conditions can be obtained as particular cases. If ^ = 0 and Ci = C2 = ht then we have a convective type boundary condition. ht{> 0) is called 'heat transfer coefficient' and has dimensions [Power] / [Temperature] [length]^. If /3 = 0, Ci = 0 and C2T0 = g{t), then we have a flux-prescribed boundary condition. Flux prescribed boundary conditions are known as type II boundary condition. Convective type boundary conditions are called type III boundary conditions and radiative-convective boundary conditions are called type IV boundary conditions. More details about these boundary conditions can be found in [24]. These four types of boundary conditions are commonly prescribed in heat transfer problems. (B) Non-standard boundary conditions (BI) Non-local boundary condition To explain the nature of a non-local boundary condition, we consider a simple onedimensional problem of fluidized-bed coating. Our interest in this problem is more on pointing out the characteristic features of a non-local boundary condition than its rigorous formulation. A thin metallic plate of thickness 26, area Ap and the initial temperature To (To > Tm) is immersed vertically in a pool of plastic coating material [29] which is maintained at temperature Too which is also the temperature away from the boundary layer. The softening temperature of the coating material is Tm > Too- Because of the high thermal conductivity and finite heat capacity of the metallic plate, the plate can be treated as a lumped parameter system in which the temperature can be regarded as a function of time only. Let Tp{t) be the temperature of the plate. In the one-dimensional problem considered here, let the metallic plate be situated at a: = 0. The problem is that of finding the temperature T(x, t) of the softened plastic, temperature Tp[t) and the extent x = 6{t) upto which the plastic has softened. The unknown quantities can be obtained by solving the following system of equations. dT d'^T PfCf-^ = ^ Z - ^ ' 0 < X < 5{t), t > 0,
(1.4.39)
T(0,0) = To, T(0, t) = Tp{t), T{6,t) = Tm.
(1.4.40)
m
A,
-{To - Tp(0) = httiTr, - T^) + pfCf J{T-
<
= /i,(T„ - Too) + P/CfiT^ - T^)^, =6
T^)dx,
(1.4.41)
(1.4.42)
32
The Stefan Problem and its Classical Formulation
The subscripts / and p stand for fluid plastic and plate, respectively, m stands for mass, p for the density and ht is the heat transfer coefficient. Equation (1.4.41) is the boundary condition at a: = 0 which because of the integration over 0 < x < 6 in the second term on the r.h.s. is a non-local boundary condition. The l.h.s. in (1.4.41) is the heat given out by the plate per unit area over the time interval [0, t] which is equal to the sum of the heat transported by convection into the coating material and the heat received by the fluid plastic. Equation (1.4.42) is the energy balance equation at x = S{t). (BII) Boundary condition of the fifth type Boundary condition of the fifth kind arises in the following context. Suppose the boundary of a region is in contact with a thermostat. The continuity of the temperature and of the flux at the surface of contact, say x = 1, in the case of a one-dimensional slab 0 < x < 1 can be expressed as Ti\,t)
= e(l,t)
, C^ = -K^, a t x = l, (1.4.43) ot ox where T and 6 are temperatures of the slab and of the thermostat, respectively, and C is the 'heat capacity' of the thermostat. The thermostat maintains a uniform temperature. On combining the two equations of (1.4.43), we obtain a boundary condition of the fifth type r.OT ^dT C— = -K — at X = 1. dt dx
(1.4.44)
(C) Boundary conditions with multivalued functions In this boundary condition the prescribed quantities are multivalued functions. So it is not a classical boundary condition in which the prescribed functions are assumed to be suflQciently regular functions. Such boundary conditions are suitable for weak formulations and can be described with the help of 'subdifferentials' (see (4.3.44) for its definition). The boundary condition derived below is an extension of the fifth type of boundary condition (1.4.44) in the weak form. Let Q be a bounded open domain in R^ which is occupied by both solid and liquid regions. The boundary dQ of Q is smooth, and is surrounded by a high conducting material of negligible thickness so that the temperature of the surrounding material is taken to be a function of time alone. Let the time interval be given by 0 < t < t*, for some positive number t*, and Q = fix (0,i*), Yl = dflx (0,t*), Q = QS^QL where Qs and QL are the regions occupied by the solid and the liquid, respectively. Yl — Y.L U Y.s U ULS where Y.L ^"^^ Tls ^re the boundaries of solid and liquid regions and YLS is the portion of YJ which is neither solid nor hquid. We assume that the measure of the set YLS is zero. Because of the perfect thermal contact between dVt and the outside material, the temperature is taken to be continuous and so T\^=e\Y^,
(1.4.45)
1.4 Classical Formulation of Multi-dimensional Stefan Problems
33
where T{x, t) is the temperature of Q and 9{t) is the temperature of the outside material. The heat balance on JZL gives (see [30]) 39 dT C— + Ki—+g[x,t,e)=Q,
(1.4.46)
where n is the outward normal to dVl, C is the heat capacity of the surrounding material and g{-) is the rate of heat flow from outside of Q to dVt. The heat balance on X]^ gives 39 3T C— + Ks-^+g{x,t,e)
= 0.
(1.4.47)
Define a multivalued function f as 1
ar) =
(^,^) 1
,
r>0,
, r = 0,
(1.4.48)
r <0.
If Ks > KL^ then ^ is a monotone graph. Equations (1.4.46) and (1.4.47) can be combined into a single equation 39 3T iC— + — + ig[x, t, 9) = 0, on OQ.
(1.4.49)
In (1.4.49) normal derivative of the temperature on 30. is prescribed in terms of a multivalued function. T < 0 is solid, T > 0 is hquid and T = 0 corresponds to the mushy region. Conditions of the type (1.4.49) are also called dynamical boundary conditions [31]. Weak formulation is a continuum model and in this formulation, solid, liquid and mushy regions (both solid and liquid phases are present in the mush) are modelled with the help of a single equation which is valid in the region Q in the distributional sense. Therefore the boundary conditions over different portions of the boundary should be combined into a single equation which results in (1.4.49). Stefan problems with dynamical boundary conditions have been investigated by several authors (cf. [31, 32]).
1.4.5
Conditions at the free boundary
In the Stefan problem two boundary conditions are prescribed at the free boundary. One specifies the temperature and another accounts for the energy balance. The isotherm condition in (1.4.7) is the simplest type of temperature-prescribed boundary condition and is justified only if the free boundary is planar and is in thermodynamic equilibrium. In this case, isotherm temperature is the equilibrium temperature T^. The equilibrium temperature is affected by the curvature and the surface tension in the free boundary.
34
The Stefan Problem and its Classical Formulation
This effect is known as Gibbs-Thomson effect Similarly equilibrium temperature may depend on the velocity of the free boundary and in this case the free boundary condition is known as a kinetic condition. An explanation of curvature effects and kinetic condition requires thermodynamical and metallurgical concepts and so these effects will be discussed a little later in § 2.3.2. One form of generalization of (1.4.8) is to consider the thermophysical parameters as general functions of space, time and temperature and include a heat flux acting at the free boundary. Such generahzations, at least in the one-dimensional problems, have been considered extensively and analyzed mathematically. Some of these extensions will be discussed later with respect to specific problems. Another type of generalization of free boundary condition is to consider the transport and accumulation of different forms of energies in the phase-change interface such as by heat conduction along the interface. This generalization of the Stefan condition, called generalized Stefan condition, will be discussed in § 2.4. The notion of Stefan condition is not restricted to conditions of the form (1.4.8) which arise due to heat balance during phase changes. Stefan condition can be considered in a functional form, for example, the velocity of the free boundary can be prescribed as a function of temperature, temperature derivatives, time, and S{t) (see (3.1.5)). Implicit free boundary condition In the Stefan condition (1.4.11), the velocity of the free boundary occurs explicitly. A boundary condition of this type is called an explicit free boundary condition. There are several problems in which the boundary conditions at the free boundary do not involve the normal velocity of the free boundary. For example the condition dT/dx\^^g(^^^ = 0 does not involve the term dS/dt. Such boundary conditions are called implicit free boundary conditions. In some one-dimensional problems, by using suitable transformations, implicit boundary conditions can be easily converted to an equivalent explicit boundary condition. Implicit boundary conditions will be discussed further in § 3.3.
1.4.6
The classical solution
The definition of a classical solution of problem (1.4.4)—(1.4.8) is given below. Classical solutions of other problems to be discussed later can be defined on similar lines. Definition: The classical solution of (1.4.4)—(1.4.8) is a quadruple (Ti,T2,$,^*) such that T^, V^^T^ are continuous in Uo
dTJdz),
i = 1,2 and V^T, can be similarly defined.
1.4 Classical Formulation of Multi-dimensional Stefan Problems
35
It may be noted that the existence of a classical solution is not guaranteed merely by prescribing sufficiently regular initial and boundary data satisfying the sign constraints, such as in (1.4.5) and (1.4.6). Some additional compatibility conditions discussed in later sections should also be satisfied, t^ cannot be arbitrarily taken and is related to the existence of the solution. In R^,n > 2, existence and uniqueness of classical solutions have been mostly proved only locally-in-time, this too under suitable constraints.
1.4.7
Conservation laws and the motion of the melt
There is a clear and overwhelming experimental evidence [33] that except in very early times, convective heat transfer dominates over conductive heat transfer during solidification/melting. Motion of the liquid which may arise due to natural convection, forced convection, or shrinkage/ expansion of the volume becomes more significant when the phase-change is of relatively large volumes of material. Motion of the melt can be accounted for by solving Navier-Stokes equations [34] together with the energy equation. Most equations of mathematical physics are derived on the application of the following conservation laws: 1. Conservation of mass: The time rate of increase of mass of a system is equal to the difference between the rate at which mass enters into the system, and the rate at which mass leaves the system (disregarding relativity effects). 2. Conservation of momentum: The time rate of change of linear momentum of the mass of a system is equal to the sum of all the forces acting on the system. 3. Conservation of energy: The time rate of change of energy (internal energy -h kinetic energy + potential energy) stored in the system is equal to the sum of the time rate of work of the external forces, the rate at which energy is transported into the system (heat energy, electrical energy etc.) or leaving the system across its boundaries and the, rate at which energy is produced or consumed in the system. The fourth conservation law which deals with the conservation of moment of momentum is not required in the context of this volume. When a conservation law is valid in any arbitrarily small neighbourhood of each material point, we say that the conservation law holds locally. If the melt is in motion, then in order to determine the motion of the melt, energy equation should be solved in conjunction with Navier-Stokes equations. We derive below the complete system of equations briefly. Control volume and applications of conservation laws A 'control volume' refers to a fixed region in space which encloses a fixed volume VQ of the fluid in space. Its surface AQ is called control surface, which could be of any shape.
36
The Stefan Problem and its Classical Formulation
In the Eulerian coordinate system^ the fluid is imagined to pass through this arbitrary control volume. At the same position we observe different particles at different times. In the Lagrangian coordinate system, the coordinate system is attached to the moving particle. Therefore the control volume consists of the same particles all the time. Let 0 be some 'scalar specific property' (per unit mass) such as mass, momentum, energy of the fluid. We shall consider the conservation of a scalar property 0 in the control volume. The time rate of change of the total property 0 contained in VQ is given by d_
dt
f pcpdVo.
(1.4.50)
Vo
The conservation equation for (f) is dt
; f p(t)dVo =^ - f {pV(t>) • ndAo - f J-ndAo+
f fdVo,
(1.4.51)
where V is the velocity of the fluid, n is the unit outward drawn normal to Ao,J is the surface flux of (p by diffusion from inside to outside across the surface AQ, f is the production or annihilation of 0 per unit volume in the control volume, p is the density and dot denotes the time derivative. dAo is the elementary surface area and dVo is the elementary volume. Using Gauss's theorem, surface integrals can be converted to volume integrals and the local conservation of
(1.4.52)
Conservation of mass : Equation of continuity Take 0 = 1 , / = 0 and J = 0 in (1.4.52). We obtain the equation of continuity as
1^ + div {pV) = 0.
(1.4.53)
Equation of conservation of linear momentum In order to obtain the equation of conservation of hnear momentum in the x-direction, take (j) = Vi, J = —Px = —{Pxxi-^Pxyj-^Pxzk) and take Fi instead of / in (1.4.52) where Fi is the body force in the x-direction. Vi is the x-component of the fluid velocity V and Pij are stress components. p.^ = -pS^j + Tij, z = 1,2,3 and j = 1,2,3,
(1.4.54)
P = - 3 (Pxx + Pyy + Pzz).
(1.4.55)
Here, Tij represent viscous stress components, and p is the fluid pressure. The r.h.s. of (1.4.52) in this present conservation law should represent the sum of the forces acting on the control volume which are surface forces and body forces. If g^^ is the x-component of
1.4 Classical Formulation of Multi-dimensional Stefan Problems
37
the body force per unit mass, then the equation of conservation of hnear momentum in the x-direction for constant p is given by
DVi/Dt is the material time derivative of Vi in (1.4.56). Two more equations in y and z directions can be similarly obtained by taking cf) = V2 and J — —Py = —{Pyx'i + Pyyj+Pyzk) ioT thc sccoud equation and 0 = V3 and J = —p^ for the third equation. We have
where Qy and g^ are the y and z components of the body force per unit mass. For Newtonian fluids [34] Tij = 2peij + p}6^J[exx + ^yy + e^,^), z = 1,2,3; j = 1, 2,3,
(1.4.59)
where Cij are the strain components. From (1.4.54) it is clear that rxx + Tyy + T,, = 0,
(1.4.60)
which implies that p^ =~s^^ where p. is the 'coefficient of viscosity'. Equations (1.4.56)—(1.4.58) can be written as a single equation in the vector notation. p ^ - p F -
gradp + / . v V + | g r a d d i v l 7 ,
F = ( p . , p „ ^ , ) , V = {V,,V,,Vs). (1.4.61)
Equation (1.4.61) is called Navier-stokes equation for Newtonian fluids. This equation is independent of the temperature. The equation of conservation of energy A general expression of specific (per unit mass) energy E is given by E = e-\- mechanical energy + potential energy + chemical energy.
(1.4.62)
Here, e is the specific internal energy or specific internal heat energy. Mechanical energy, potential energy and chemical energy are generally omitted in the formulation of Stefan problems and e is taken as specific enthalpy which is the sum of the latent heat and the sensible heat (see § 2.1.3 for their definitions). Let E — e = h, where h is the specific
38
The Stefan Problem and its Classical Formulation
enthalpy. Taking (j) = h and J = —KVT in (1.4.52), where K is the thermal conductivity in the isotropic case, we obtain j^iph) + V • (pVh) = V . {KVT) + / .
(1.4.63)
Here, / is the heat generated or absorbed per unit volume. V is to be obtained by solving Navier-Stokes equation and substituted in (1.4.63). To take into account mechanical and potential energies in (1.4.63), we take body force F = —grad ip, J = —KVT — Py, where Pv is given by 3
Pv = if^PikVk,
3
3
iZPikVk, Y.PzkVk).
(1.4.64)
fc=l fc=l fc=l
Here, pij is the stress tensor defined in (1.4.54). We present below the final form of the energy equation for constant p case (cf. [34]). p - ^ ( e + i|V|2 + «/.) = V(KVT)
- div(pV) + WT + f,
(1.4.65)
^T = T^{V | ( ^ - •^f x) ) ++ -^[V | ( ^ - •^f ,. ). .+, ,—{V v • T,),
(1.4.66)
-?i = iTix + JTiy + kTiz, i = x,y, z.
(1.4.67)
Here, jplVI^ is the kinetic energy and for simplicity we take the body force to be F = — grad ij).
Chapter 2 Thermodynamical and Metallurgical Aspects of Stefan Problems 2.1
Thermodynamical Aspects
Several generalizations and extensions of the two-phase problem (1.4.4)—(1.4.8) are possible but before dealing with them, we give some definitions. Many terms like equilibrium temperature, thermal conductivity, latent heat, enthalpy, etc., have been used earlier without defining or explaining them for the reason that greater concern was shown for the mathematical formulation. Some thermodynamical aspects of Stefan problems will be discussed in this section.
2.1.1
Microscopic and macroscopic models
A system is called microscopic if it is of 'small dimensions', roughly of the size of an atom or a molecule, i.e., of the size of 10~^^ meters. In a microscopic model, thermal, mechanical or chemical phenomenon is studied at an atomic or molecular level. In a macroscopic model, the system is 'large enough' of the order of one micron, be visible with a microscope using ordinary light. A macroscopic system consists of large numbers of atoms or molecules. There is an important difference between a microscopic model and a macroscopic model. In the microscopic model, the description of an individual particle motion/action, even if available would not disclose the gross behaviour/properties of the system. For example, it is a striking fact, and one which is difficult to understand in microscopic detail that simple atoms forming a gas can condense abruptly to form a liquid with very different properties. It is well known that a transport phenomenon whether it is electrical, heat or mass transfer, occurs due to changes in the energy levels of atoms or molecules. Imagine applying Newtonian laws of motion to 10^° molecules and obtaining information for each molecule in a transport phenomenon such as heat transfer. It is an awesome task. All the 39
40
Thermodynamical and Metallurgical Aspects of Stefan Problems
mathematical formulations discussed earlier are based on macroscopic modelling. Does it meant that microscopic modelling is not required? However complicated, microscopic models cannot be totally discarded. Kinetic theory of matter (cf. [35, 36]) applies the laws of mechanics to individual molecules of a system and enables one to calculate, for example, the numerical values of heat capacities, heats of transformation, viscosity. These parameters can be explicitly defined in a macroscopic model, but their numerical values can be obtained only on the basis of a molecular model. As stated earher some of the equations of mathematical physics, for example, the heat equation can be easily derived on the basis of a microscopic model. The advantage of the macroscopic model or a continuum hypothesis, and in which we are interested is that the gross behavior of the system can be studied and laws of Newtonian mechanics can be applied to the bulk matter. An approach more general than macroscopic modelhng is of statistical thermodynamics which, ignores the detailed consideration of molecules as individuals, and applies statistical methods to find the distribution of very large number of molecules that make up a macroscopic piece of matter over energy states of the matter. The equations of conservation of mass, momentum and energy arising in continuum mechanics can be obtained as particular cases of the Boltzmann equation [36]. Both kinetic theory and statistical thermodynamics were first developed on the assumption that the laws of mechanics deduced from the behavior of matter in bulk, could be applied without change to particles like molecules and electrons. As science progressed, it became evident that at least in some respects this assumption was not correct, that the conclusions drawn from it by logical methods did not agree with experimental facts. For example, experiments suggest that the specific heat of many solids at constant volume approach the Dulong-Petit [35] value of 3R {R is universal gas constant) at high temperatures, but decreases to zero at very low temperatures. This behavior of solids can be explained with the help of a quantum mechanics approach. The failure of small scale systems to obey the same laws as large scale systems led to the development of quantum theory and quantum mechanics. Statistical thermodynamics is best treated today from the view point of quantum mechanics. On a microscopic scale classical mechanics does not apply and must be replaced by quantum mechanics. For further details of quantum theory, the reader is referred to [35, 36, 37].
2.1.2
Laws of classical thermodynamics
We shall be deahng here mostly with equilibrium thermodynamics. Thermodynamics is the study of energy and its transformation. There are many different types of energy but most studies of thermodynamics are primarily concerned with two forms of energy: heat and work. Thermodynamics deals with the macroscopic properties of matter and is an empirical science. It is developed on a small number of principles which are generalizations made from experience.
2.1 Thermodynamical Aspects
41
Thermodynamic equilibrium: When a system is in thermal, mechanical and chemical equilibrium, it is said to be in thermodynamic equilibrium. In thermal equilibrium, the temperature will be the same at all points of the system. In mechanical equilibrium, all motions, expansions or contractions of the system are absent. Note that atoms are still in motion. When all the chemical reactions stop then the system is in chemical equilibrium. Reversible and quasi-static process: A process is called reversible if the initial state of the system can be restored with no observable effects in the system and its surroundings. If a process is not reversible, it is called irreversible. If a process is carried out in such a way that at any time the system departs only infinitesimally from the equilibrium state it is called a quasi-static process. First law of thermodynamics: Internal energy (internal heat energy) of a system is the sum of all the individual kinetic energies of motion and energies of interaction (potential energies) of the particles in the system. Internal energy can be transformed to do work and produce heat. One form of the first law of equihbrium thermodynamics is dU = d'Q-d'W.
(2.1.1)
Here, dU is the change in the internal energy of the system from equilibrium state a to another equihbrium state 6, d'Q is the heat flow into the system during the change of state and d'W is the work done by the system when the system changes its equilibrium state from a to b. The dash indicates that the quantities are not exact differentials. Internal energy is a state property, i.e., internal energy in state b does not depend on the process by which the system has been brought from state a to state b. dU is an exact differential but not d'Q and d'W. It can be easily shown that work is path-dependent and so is heat flow [35]. Equation (2.1.1) holds for both reversible and irreversible processes. Second law of thermodynamics: Entropy Some changes in a system can take place only in one direction. Consider an isolated system in which a body at temperature Ti is in contact with a heat reservoir at temperature T2 > Ti. Heat will flow from the reservoir to the body and raise its temperature to T2. Is it possible for the body to cool down to temperature Ti by releasing heat to the reservoir? The change in this direction is not possible. It may be noted that the total energy of the system consisting of the body and the reservoir is conserved even if a reverse change takes place. Therefore if we are looking for some property of the system whose change can tell us the direction in which the reverse change is possible, then it cannot be energy. This property of the system is called entropy, denoted by S and defined as / d'Q dS = S2 — Si = / -—, (reversible changes) .
(2.1.2)
1
Here, 1 and 2 are the two equilibrium states, of a system. Entropy is defined only for reversible processes. The second law of thermodynamics, states that processes in which
42
Thermodynamical and Metallurgical Aspects of Stefan Problems
the entropy of an isolated system (i.e., dQ — 0) would decrease do not exist. Specific entropy (entropy per unit mass) will be denoted by s. If hot water is mixed with cold water, then the entropy of the cold water will increase more than the decrease in the entropy of the hot water which can be checked from (2.1.2). So there will be an increase in the entropy of the system consisting of both cold and hot water. This tells us that heat cannot flow from cold water to hot water as decrease in the entropy of an isolated system is not possible. Unlike energy or momentum, entropy is not conserved. The first law of thermodynamics states that energy can neither be created nor destroyed and the second law states that entropy can not be destroyed but it can be created. The process is called isentropic if the entropy of the system does not change during the process.
2.1.3
Some thermodynamic variables and thermal parameters
Specific heat capacity or specific heat: If no phase-change takes place in a process, then the heat capacity C at any temperature is defined by the equation d'Q = CdT,
(2.1.3)
where d'Q is the quantity of heat added to the system which changes its temperature by dT. The process could be reversible or irreversible. If the volume of the system does not change in the process, then C is denoted by Cy and is called heat capacity at constant volume. At constant volume, dV = 0. Prom (2.1.1), we obtain
'•"-^w),"''^'^"-^^^-{%)M{whr
*"•">
where the only work done is by the pressure P in changing the volume. From (2.1.3), we get
C,^{%l.
(.1,5,
It may be noted that Q is not a function of temperature. Therefore C is not the derivative of Q with respect to T but is only the ratio of d'Q and dT or the ratio of very small amount of heat supplied and the change in the temperature. A specific value of an extensive property (a property such as energy which depends on the mass) is its value per unit mass. Specific heat is defined as the heat capacity per unit mass and will be denoted by C. Enthalpy:
If the pressure P does not change in the process, then from (2.1.1), we
get dU = d'Q - PdV.
(2.1.6)
2.1 Thermodynamical Aspects
43
Also dU + PdV = dU-{- PdV + VdP = dU-^ d{PV) = d{U + PV).
(2.1.7)
Let H = U -\- PV, where P is constant. H is called enthalpy and specific enthalpy will be denoted by h. H is a. function of state, or a thermodynamical property. In this volume, H has also been used for enthalpy per unit volume. If a thermodynamic quantity depends only on the state of the system and not on the process by which it has been brought to that state, then it is called a thermodynamical property. Temperature and pressure are examples of thermodynamical properties. From (2.1.6) and (2.1.7), we have dH = dQ = CpdT,
(2.1.8)
Here, Cp is the heat capacity at constant pressure. From the relation dH = dQ (at constant pressure), it is clear that enthalpy can be regarded as the heat content of the system. A positive AH (change in H) means that heat is absorbed while a negative AH means that heat is released. Gibbs free energy and Helmholtz free energy: We now ask 'Is it possible to describe the thermal equilibrium of a system in terms of some property of the system itself even if it is in contact with surroundings' ? A system and its surroundings together is called a 'universe'. Universe is considered to be isolated and closed. For an isolated system, d'Q = 0 and so dS = 0. Let dS and dS^ be the changes in the entropies of the system and its surroundings, respectively, then from the second law of thermodynamics (dS^dS,) >0.
(2.1.10)
Let the system absorb some heat d'Q from its surroundings so that the internal energy of the system changes from U to U -\- dU and the system does some work d'W = PdV in a ^PVT system'. If T is the temperature of the system and its surroundings, then dS, = -d'QIT
and [dS - d'Q/T) > 0.
(2.1.11)
Substituting d'Q from (2.1.1) in (2.1.11), we get dU + PdV -TdS
<0.
(2.1.12)
All the quantities in (2.1.12) belong to the system. In any reversible process between two equilibrium states, dS + dS^ = 0 and so dU + PdV -TdS
= 0.
(2.1.13)
44
Thermodynamical and Metallurgical Aspects of Stefan Problems
If P and T are constant, then d{U + PV - TS) = dU + PdV - TdS = 0.
(2.1.14)
Therefore, the quantity U + PV — TS is a function of the state of the system. It is called Gibbs free energy denoted by G and G=U-{-PV~TS
= H-TS,
H = U + PV.
(2.1.15)
Note that G is the free energy of the system and not the free energy per unit volume. Interestingly in deriving (2.1.15) we had assumed that the process is reversible. But G is a function of the state as all the quantities on the r.h.s. of (2.1.15) are functions of state. It does not matter whether the state of the system is changed by a reversible process or by an irreversible process. Equation (2.1.15) is very general and apphes to a system of any nature. The process may be a change of state, or a change of phase, or a chemical reaction. Suppose the temperature of the system rises by an amount dT under constant pressure. When the system changes from one equilibrium state to another then from (2.1.15), we get dG = dH-
TdS - SdT.
(2.1.16)
Prom (2.1.3), dQ = CpdT (under constant pressure d'Q = dQ) and therefore from (2.1.2) dS = CpdT/T.
(2.1.17)
dG=-SdT.
(2.1.18)
Since dH = dQ = CpdT, we have
The free energy decreases with the rise in temperature at constant pressure. The Gibbs free energy is the most useful of all thermodynamical properties and provides a practical criterion for a system to be in thermodynamical equilibrium. If a system is in thermodynamical equilibrium, then dT = 0 and dG = 0.
(2.1.19)
Out of the infinitely many states in which a system can exist, the equilibrium state is the one whose free energy does not change in any process which is carried out under constant pressure and constant temperature. dG = 0 also implies that in an equilibrium state, free energy would be minimum. Let us now consider a system at constant temperature and constant volume. It is easy to see that d(U - TS) - dU - TdS = 0.
(2.1.20)
2.1 Thermodynamical Aspects
45
The quantity F = U — TS is called Helmholtz free energy. F is also a function of state and plays the same role at constant temperature and constant volume as Gibbs free energy plays at constant temperature and pressure. dF can be expressed as dF = -SdT - PdV.
(2.1.21)
Latent heat: When a solid metal piece is heated, the temperature rises and reaches an equilibrium temperature Tm (see the next section). When more heat is supplied, the heat is first absorbed without raising the temperature of the piece. The heat so absorbed is called latent heat of melting. It is measured in terms of per unit mass and is the ratio of the heat absorbed and the mass m undergoing the change of phase. Why is heat absorbed ? In solid metals, the free atoms are closely packed. Atomic arrangement is disordered in liquids in comparison to that in solids. So the latent heat of fusion (or melting) is the energy required to pull the atoms apart to the more openly packed structure of the liquid [38]. In general when a phase changes isothermally from solid to liquid or liquid to vapour or solid to vapour, the system absorbs heat. The heat absorbed in these three transformations is called latent heat of fusion, latent heat of vaporization and latent heat of sublimation, respectively. When a liquid solidifies latent heat is released. Latent heat released per unit mass is taken as positive, and is denoted by /. Latent heat of solidification is the negative of the latent heat of fusion. Thermal conductivity: Thermal conductivity is a transport property and is not a function of state. Transport of heat energy takes place by molecular mot ion/vibration and is called heat conduction. Heat conduction in equilibrium thermodynamics takes place according to an empirical law called the law of Fourier's heat conduction. For isotropic bodies this law is given by (1.3.8) and for anisotropic bodies by (1.4.9).
2.1.4
Equilibrium temperature; Clapeyron's equation
Melting or freezing temperature: The free energy curves vs temperature (pressure constant) for solid and liquid phases of a metal are given in Fig. 2.1.1. Since free energies of the two phases are the same at the temperature T^, the two phases can stay in equilibrium at Tm as no exchange of free energy can take place. Tm is called equilibrium temperature. Tm is also called melting or freezing temperature. It can be called an ideal equilibrium temperature as the phase-change takes place at Tm under ideal conditions which ensure that the phase-change boundary is planar. In alloys phase-change takes place over a temperature range. If the temperature of the liquid is less than Tm, the hquid is called supercooled. It is clear from Fig. 2.1.1. that for T < Tm, the free energy of the sohd is less than the free energy of the hquid. Therefore sohd phase is a stable phase for T < Tm and supercooled liquid is not a stable phase. For a solid, the specific enthalpy h is given by h = CsT, T
(2.1.22)
For a liquid h = CLT^l
/ > 0 ; T>Tm.
(2.1.23)
46
Thermodynamical and Metallurgical Aspects of Stefan Problems
AtT = Trr., he[CsTm.CLTm
(2.1.24)
+ l]
Crystal
0"K
Tm Temperature
Fig. 2.1.1. Free energy vs temperature Prom (2.1.15), AH = TmAS as AG = 0 at T = T^, where AH and AG stand for the difference between their values in the two phases at T = T^. Therefore
U{CL-Cs)Tm_
As:
I
(2.1.25)
where As is the specific entropy of the hquid minus the specific entropy of the sohd at T =T Clapeyron's equation: When two phases have different specific volumes their temperature of mutual equiUbrium depends on the pressure as the phase-change causes work PdV to be done. Let Gi(T, P) and G2{T, P) be the Gibbs free energies of the solid and liquid phases, respectively. Since the phases are in equilibrium (2.1.26)
G i ( T , P ) - G ' 2 ( T , P ) = 0,
dT
dT r^
^[ dP
dP ' "^
^'
On using the definitions of Gi and G2 and (2.1.25), we obtain dP dT where V stands for the volume.
1 + {CL- Cs)Tm (KL - Vs)Tm
(2.1.27)
2.2. Some Metallurgical Aspects of Stefan Problems
2.2 2.2.1
47
Some Metallurgical Aspects of Stefan Problems Nucleation and supercooling
It is a common experience that when most hquids are cooled sufficiently, they become solid in course of time. A liquid metal normally solidifies by crystallization. In addition to the solid state, metals also exist in a glassy or an amorphous solid state, a state which is hard but not crystalline. The first step in the growth of a crystal is nucleation. A small nucleus appears in the liquid phase which then grows by the addition of more material from the liquid phase. The formation of a rain drop in a cloud is a familiar example. The theory of 'nucleation' is important not only in crystal growth but also in understanding supercooling/superheating and supersaturation effects. Some authors use the term undercooling in the place of supercooling. A nucleus may form at the surface of the mold or inside the melt. When the probability of forming a nucleus is the same anywhere in the melt, then the system is in a state suitable for homogeneous nucleation. If some sites are more suitable for nucleation than others, then the system is in a state suitable for heterogeneous nucleation. In nature most nucleations occur heterogeneously. For heterogeneous nucleation we refer the reader to [39]. How does nucleation occur ? A phase transformation involves rearrangement of atoms and requires some irreversible departure from equilibrium. Phase transitions are driven by thermal fluctuations and in liquid metals, random fluctuations may create minute crystalhne regions called 'clusters' or 'embryos' even at temperatures higher than T^. On this already existing new phase which has grown randomly some more material of the new phase should grow for a crystal formation. Some sites such as rough surfaces of the mold, foreign bodies in the melt, and seeds of the new phase added to the melt help the process of nucleation. Further details of heterogeneous nucleation or even homogeneous nucleation are too complicated to be presented here. For a nucleus to grow, some work has to be done by the system which results in a change in the free energy AC (per unit volume) which is the difference between the total free energy of the new phase (solid), and the total free energy of the old phase (liquid). Total free energy is the sum of the bulk energy and the surface energy, and is given by AG = r^AGv + ar^a,
(2.2.1)
where r is the linear dimension of the nucleus and r"^ its volume, AGy is the change in the bulk free energy of the volume, ar^ is the surface area of the nucleus and a is the surface energy per unit area, a is generally positive. AGy is negative as the enthalpy of the liquid is greater than that of the solid. When the radius of the nucleus is very small, the second term on the r.h.s. of (2.2.1) dominates the first (a is large when the nucleus is very small) term making AG positive. AG reaches its maximum when at r = TQ (see Fig. 2.2.1.) and
dr
Thermodynamical and Metallurgical Aspects of Stefan Problems
48
Using (2.2.1) in (2.2.2), we obtain r = ro =
-2aa/{3AGv) and A = AG\r
4aV^ 27{AGvy
(2.2.3)
Since at equilibrium AGy = 0, AG becomes very large near r = TQ. The nucleus cannot grow beyond r = TQ until this large energy barrier is crossed, for example by artificial seeding with the fragments of the new phase. Once this barrier of energy is crossed, AC starts decreasing and nuclei are formed when AG is negative. The critical size r = ro of the nucleus corresponds to an unstable equilibrium between the nucleus and its parent phase; AC makes larger one grow and smaller one shrink.
Fig. 2.2.1. AC, AGy and surface energy vs r Supercooled state: A state in which the temperature of a liquid is below the ideal equilibrium temperature Tm is called a supercooled state and the liquid is said to be supercooled. If the temperature of the solid is greater than T^, the solid is said to be superheated. It may be mentioned that the term supercooling defined here is different from the constitutional supercooling which arises in alloy solidification and in which the freezing temperature may decrease with the rise in concentration. Degree of supercooling For simplicity the formation of a single nucleus in homogeneous nucleation will be considered. Let T be the nucleation temperature. From (2.1.18) and (2.1.25), we have AH d{AGv In (2.2.4) -AS=dT T VmTm Here, Gy is the free energy per unit volume. It has been assumed in (2 2.4) that C 5 = CL, a small change in the entropy due to a temperature change can be ne;fleeted and Tm can be used in the place of T. Im is the latent heat per mole and Vm is the molar volume, The 'mole' or the 'mole number' of a substance is defined as the ratio of its mass and the molecular weight and the 'molar volume' is the ratio of volume and the mole number. Using Taylor's series expansion for AGy, we obtain (AGv
(AGv
+ (T - r,„
.d{AGy' clT
\T„,
+ ..
2.2. Some Metallurgical Aspects of Stefan Problems
=
- ( T . - r ) ! ^ | . „ = -
49
%
^
.
(2.2.5)
In deriving (2.2.5) we take {AGV)\T^ = 0 as at T = Tm the bulk free energies of solid and liquid phases are the same and higher order terms have been neglected. If T < T^ in (2.2.5), then {AGV)T is negative. For large radius of the nucleus, AG can be taken to be equal to AGy as the second term in (2.2.1) is very small. If the phase-change from liquid to solid takes place at a temperature lower than Tm, then the liquid is supercooled and {Tm — T) gives the degree of supercooling. Note that AG is negative, which it should be for a crystal growth. If a system exists in a state of its lowest fee energy, then the system is in a stable state and it can remain in this state for a long time. However, systems often exist for long times in states not of lowest free energy, for example, a mixture of oxygen and hydrogen gases can stay unchanged at room temperature even though the stable state is water. Such systems are said to be metastable. Supercooled liquid and superheated solids are in metastable states and if they get some extra energy to rearrange a group of atoms into a next and more stable phase, the phase-change to a stable phase will take place. The stable state for a supercooled liquid is solid and for a superheated solid, it is liquid. The extra energy can be provided to the supercooled liquid, for example, by artificial seeding. During solidification, latent heat is released by the liquid which warms up the growing crystal and the solid-Hquid interface attains the temperature Tm (see Fig. 2.3.7.). Supercooling is generally small but can be as large as 0.2 Tm. For typical metals Tm ~ lOOO/C to 2000X. It may be noted that Tm is actually an ideal freezing temperature as the phase-change from liquid to solid generally takes place at a temperature lower than Tm- This is because of the effect of interface curvature and the kinetic undercoohng which are discussed in the next section.
2.2.2
The effect of interface curvature
In the crystallization of a pure metal, at the interface between nucleated crystal and its melt, there is considerable activity amongst the atoms. This is because of the small amount of activation energy required to change the phase. In simple words, activation energy can be described as the least amount of energy required to change a metastable phase to a new and more stable phase. Some atoms break away from the crystal at the surface to join the liquid and some atoms in the liquid crystallize on the surface and become part of the crystal. If the rates of these two opposing processes are equal, then the surface is at the crystal-liquid equilibrium temperature. This equilibrium temperature need not be the ideal equilibrium temperature Tm- If the surface of the crystal is curved, and the center of curvature lies inside the crystal, then on an average the atoms at the surface are less surrounded by neighbouring atoms of the crystal than otherwise and their escape tendency into the liquid increases. Less energy is required for a phase-change and phase-change takes place at a lower temperature. The equilibrium temperature is lower than the ideal value Tm [40]. Conversely, if the center of curvature lies outside the crystal, then the equilibrium temperature will be higher than Tm- These effects are small unless
50
Thermodynamical and Metallurgical Aspects of Stefan Problems
radius of curvature is small. The deviation AT from the ideal equihbrium temperature Tm (which is the temperature of a planar phase-change interface) is given by AT = T ^ - T „ = A'ero = ^ ^ ^ f y ^ ,
(2.2.6)
where T^ is the new equilibrium temperature due to the curvature effect, Kc is the sum of the principal curvatures of the curved surface, a is the surface energy per unit area and Im is the latent heat per mole. Note that the curvature Kc and the constant FQ are defined in such a way that a positive undercooling, i.e., a decrease in the equilibrium temperature is associated with a portion of solid-liquid interface which is convex towards the liquid phase (see Figs. 2.4.2. and 2.4.3.). Kc is negative in this case. Equation (2.2.6) is called Gibbs-Thomson relation. The relation (2.2.6) can be obtained as follows. Consider a small solid particle (crystal) in the melt. The sum of the principle curvatures of the curved surface of this particle is denoted by Kc- Let AGr be the difference between the total free energies (per unit volume) of the new phase (very small crystal) and the old phase (liquid) when the curvature effects are included in the phase-change temperature T^. Note that the total free energy in any phase is the sum of the free energy of the bulk and the surface energy. If the volume of the liquid is much large than the volume of the crystal, and if \T^—Tm\ is not very large, then it can be reasonably assumed that in solidification the free energy of the liquid does not change due to the curvature effect. In this case as discussed below AGr can be regarded as the difference between the total free energies of a very small crystal (with curvature) forming at temperature T^ and a very small crystal forming at Tm without curvature. AGr = total free energy of crystal at T^ — total free energy of liquid at T^ (2.2.7) = total free energy of crystal at T^ — total free energy of liquid at Tm = total free energy of crystal at T^ — total free energy of crystal at T^. (2.2.8) Since free energies of solid and liquid at Tm with no curvature are equal, we get (2.2.8). I AGr I can also be calculated with the help of (2.1.15) and (2.2.8) and we get |AG,| = \AH - r ^ A 5 | = |A5||(T„ - T J | .
(2.2.9)
Here, modulus of different quantities has been taken for convenience. A5 is the difference between the entropies (per unit volume) of solid and hquid at the temprature T^. It can be shown [40] that if Cs = CL then {AH)T^T^
= {AH)T^Tm-
(2.2.10)
The change in the free energy of the crystal due to curvature or the change in the free enthalpy of the crystal (per unit volume) due to curvature is due to the change in the internal pressure [40] and so | A a | = |AP|.
(2.2.11)
2.2. Some Metallurgical Aspects of Stefan Problems
51
The internal pressure AP is given by the relation [40] \AP\ = a\K,\.
(2.2.12)
When Gibbs-Thomson law is written as r ; = r„ + ^ ^ f y ^ ,
(2.2.13)
then by convention Kc is taken as negative if the phase-change interface is convex towards the liquid, i.e., T^ < Tm as a, Tm and Im are taken as positive. If T^ > T^, then from (2.2.4) (2.2.9), (2.2.11), and (2.2.12) (modulus sign can be removed as we are dealing with positive quantities on both sides), we get Tm-T'm = crKcTmVm/lm, K > 0,
(2.2.14)
which is the relation (2.2.6).
2.2.3
Nucleation of melting, effect of interface kinetics, and glassy solids
Nucleation of melting: Melting of solid can begin without significant superheating in the solid. When nucleation of melting occurs over a sohd surface, the nucleus consisting of the liquid phase will be enveloped basically by two different surfaces. On one side is a sohd-hquid interface (see Fig. 2.2.2.) and on the other side is a gas-hquid interface. The surface energy of the gas-solid interface is generally greater than the combined gas-liquid and solid-liquid surface energies. This implies that the formation of the liquid film on the solid surface will not be opposed by the solid surface and the liquid film spreads over the solid surface without significant superheating. The equilibrium of surface energies is attained at the equilibrium temperature. Effect of interface kinetics: It was suggested in [41], that the interface movement lowers the temperature of the solid-liquid interface below the equilibrium teriiperature TmThe form of the growing pheise adapts itself to satisfy the growth velocity-temperature requirement. The movement of the interface is dependent on a driving force which was called the viscous effect or the interface kinetic effect. The supercoohng as a function of interface velocity can be expressed as T(V) = Tr,-$-V,
(2.2.15)
where T{V) is the new melting temperature, V is the velocity of the interface and (p is the viscous correction term. For small departures of T{V) from T^ the rate at which solidification occurs is approximately proportional to the departure [42]. In high transition rates these effects appear on a macroscopic scale also. Kinetic undercooling is likely to be important only when V • n (normal velocity) is large, such as V - n ^ lOm/s. The applicability of kinetic effect to a moving interface has been opposed by some authors [43] as the concept devolves from the thermodynamics of equilibrium systems which has been
52
Thermodynamical and Metallurgical Aspects of Stefan Problems Gas-solid Interface
Liquid
Gas-liquid Interface
Solid-liquid Interface
Solid
Fig. 2.2.2. Nucleation of melting applied in (2.2.15) to transforming systems. The interfacial free energy of the solid-liquid interface is a thermodynamic quantity only in the limited context of stationary interfaces which are at equilibrium. See [42] and [44] for further information. Glassy (amorphous) solids: At very high cooling rates, such as those encountered in rapid solidification of liquid metals, there may be insufficient time for the formation of nucleus and a glassy solid is formed. While still non-crystalline, it becomes a solid in its mechanical behaviour as its viscosity becomes very high. There is no phase-change boundary associated with glassy materials.
2.3
Morphological Instability of t h e Solid-Liquid Interface
A crucial assumption in the classical formulation of Stefan problems is that there exists a smooth surface which is the phase-change boundary such that on one side of this surface is a stable solid (liquid) region and the other side a stable liquid (solid) region. The solid region is identified only by its temperature which should be less than or equal to the equilibrium temperature T^. Similarly liquid is identified only by its temperature which should be greater than or equal to T^. The existence of such a planer solid-liquid interface is an ideal situation and is possible only in special cases such as columnar solidification of pure metals or directional solidification of alloys in 'Bridgman type furnace' [40] under a sufficiently high temperature gradient. In unidirectional solidification, in its classical formulation, it has been established mathematically in several references such as in [45] that under certain conditions, such as, in the absence of volumetric heat sources, or non-existence a mushy region initially, a planar interface remains planar (mushy region does not develop) till the solidification is complete. In multi-dimensional problems, the existence of a sharp interface has been proved rigorously only for a short-time under suitable assumptions (see Chapter 10). Invariably, a sharp interface separating a stable solid region from a stable liquid region
2.3 Morphological Instability of the Solid-Liquid Interface
53
degenerates after a short time into a mushy region. The mushy region contains both sohd and hquid phases and separates the stable sohd region from the stable liquid region. The solid in the mushy region is present initially in the form of dendrites (Figs. 2.3.1. and 2.3.2.). Further growth of dendrites leads to a poly crystalline structure with one crystal from each nucleus (Fig. 2.3.3.). The diameter of a grain could be as smah as 10"^m and as large as 10~^m. Mushy region may also develop during solidification if the hquid is supercooled or during melting if the sohd is superheated. This degeneration of sharp interface into a mushy region is attributed to the morphological instability of the solidliquid interface.
Fig. 2.3.1. Formation of dendrites
Fig. 2.3.2. Dendrite growth
Classical thermodynamic definitions of stability are inapplicable to the determination of the morphology of a growing interface, and current extensions of equilibrium thermodynamics have not yet furnished a fully acceptable alternative [46]. On the basis of a heuristics-based stabihty criterion, the interface is said to be morphologically unstable/stable if a small perturbation given to the interface grows/dies with time (see Fig. 2.3.4.). In Fig. 2.3.4. (a), the perturbations are growing, making the planar interface
Fig. 2.3.3. Formation of polycrystalline structure.
Thermodynamical and Metallurgical Aspects of Stefan Problems
54
Fig. 2.3.4. (a) Small perturbations at the interface are growing (b) Perturbations are dying. unstable. Mushy region will develop in front of the interface. In Fig. 2.3.4. (b), the perturbations are dying and the planar interface remains planar. Figs. 2.3.5—2.3.7. show solid-liquid interface morphology and the temperature distributions in the solid and hquid regions. In Fig. 2.3.5., freezing temperature is equilibrium temperature T^. The interface is planar and perpendicular to the direction of heat flow. The hottest part of the system is the hquid region. In Fig. 2.3.6. the growth of freezing front is columnar. The hottest part of the system is liquid. This growth is more common in alloys. In Fig. 2.3.7., the grains are equiaxed and have similar dimensions along all axes. The hottest part of the system is the crystal as the liquid is supercooled. The interface is stable in Fig. 2.3.5 and unstable in Figs. 2.3.6. and 2.3.7.
+ ;I
-q" 0 D 0)
a E
/ ^
Solid ^*r-q
Liquid -«—q'
(D
Interface-^
Fig. 2.3.5. Solid-Liquid interface morphology and the temperature distribution. Planar growth.
Fig. 2.3.6. Solid-Liquid interface morphology and the temperature distribution. Columnar growth.
2.4 Non-material
Singular Surface: Generalized
Stefan
Condition
55
Fig. 2.3.7. Equiaxed dendrite growth
2.4
Non-material Singular Surface: Generalized Stefan Condition
A singular surface is a surface which does not have any inner structure so as to give rise to discontinuities in the bulk quantities. SoHd-hquid interface and shock waves are examples of singular surfaces. The motion of a singular surface could be independent of the motion of the bulk phases. A material surface is a surface which consists of the same material particles all the time. Fluid films and coatings are examples of a material surface. A surface which is not a material surface is called a 'non-material surface'. It is well known t h a t surfaces of bodies, and interfaces between pairs of bodies, exhibit properties quite different from those associated with their interiors. There have been several a t t e m p t s (cf. [46, 47, 48, 49]) at formulating the equations of conservation of mass, motion and energy in a moving singular surface embedded in a three-dimensional continuum and dividing the continuum into two regions for which the singular surface is a common boundary. Some thermodynamic fields may have discontinuities across this singular surface. In the classical Stefan condition (1.4.8), the j u m p in the normal component of the heat flux vector across the phase-change boundary is given in terms of the amount of latent heat released or absorbed at the phase-change boundary. It is assumed t h a t no mechanical work is done. The modern theory of surface thermodynamics and mechanics takes into account other aspects also such as the surface stress, in particular surface tension, and conduction of heat tangential to the surface while balancing energy along the singular surface. The Stefan condition derived below takes into account the surface tension and heat conduction while balancing energy in the phase-change boundary, and will be called generalized Stefan condition [51]. Consider a three-dimensional bounded region R which is occupied by solid and liquid
56
Thermodynamical and Metallurgical Aspects of Stefan Problems
phases. There exists a time dependent smooth singular non-material surface F which divides R into two parts Rs and RL, R = Rs ^ RL ^ ^• Rs and RL are solid and liquid regions such that Rs H Ri is empty. The subscripts S and L stand for the solid and the liquid phases, respectively. The external surfaces of Rs and RL will be denoted by dRs and dRi, respectively (see Fig. 2.4.1.). Let (Rs)b = dRsUT and (RL)^ = dRiUT. The external surface of R is dRs U dRi U dT where the curve dV is the boundary of F. The surface F is orientable and the unit normal A^ to F points into the liquid phase. Let V be the velocity of F in the direction of TV, i.e., V = \V\N. \i may be noted that only the normal component of the velocity of the interface can be determined as in (L4.15). The tangential component of the interface motion for a massless interface cannot be derived a priori by recourse to principles of general vahdity [52]. We shah first balance the forces in R. These results will be used in balancing energy later. Conservation of forces in R fpBdv-\-
f Tnda+
f fiydl = 0.
dRsUdRL
(2.4.1)
dr
Here, B is the body force per unit mass, n is the unit normal vector pointing outwards as the exterior boundary of a region is traversed in the anticlockwise direction, r is the bulk stress tensor and f is the surface stress tensor, and /7 is the outward unit vector normal to dT which is tangential to F. dv, da and dl are elementary volume, elementary area and elementary length, respectively. We take ps = pi = P for simplicity. / 3
3
3
\
\i=l
i=l
1=1
/
( m , 712,723),
(2.4.2)
Tij, z = 1, 2, 3, J = 1,2, 3 are components of bulk stress tensor r . If it is assumed that both solid and liquid regions are in equilibrium independently of each other, the balancing of forces in the solid region gives / pBdv + / Tsfida + / TsNda = 0. Rs
dRs
(2.4.3)
r
Similarly balancing of forces in the liquid region gives / pBdv + f Tinda - / TiNda = 0. RL
ORL
(2.4.4)
r
The minus sign in the last term of (2.4.4) arises because the outward normal to the liquid region at the boundary F points in the direction opposite to TV. Adding (2.4.3) and (2.4.4) and on subtracting the sum from (2.4.1), we obtain / TLNda - / TsNda + / rudl = 0. r r dr
(2.4.5)
2.4 Non-material Singular Surface: Generalized Stefan Condition
57
Applying surface divergence theorem to the last term of (2.4.5) gives integration over r and then assuming local equilibrium, we obtain (TL - Ts)N + divrr = 0 in T.
(2.4.6)
Here, divpT is the surface divergence of r (cf. [47]). Surface stress tensor generahzes the usual notion of surface tension and is consistent with the atomistic calculations, indicating the presence of compressive surface stresses in certain crystals [47]. If f = crl, where a is a scalar valued function called surface tension and 1 is the inclusion mapping from the tangent space of F into the three-dimensional vector space [47], then (TL - Ts)N + K^aN -h Vpa = 0, in T.
(2.4.7)
Here, Kc is the sum of principal curvatures of the singular surface. The surface divergence of a smooth vector field u : T -^ G, where G is the translation space of a finite-dimensional Euclidean point space, is given by divrw = trDu,
(2.4.8)
where the r.h.s. in (2.4.8) is the trace of the tangential derivative Du of u [47]. Note that u admits a unique decomposition, u = us -\- u^N, where its is a vector in a smooth tangential field and Un is a scalar in a smooth scalar field. If r is a smooth tensor field, then for any scalar field (j) divr(0r) = (p divpr -h r V r 0 .
(2.4.9)
Taking (I) = a and r = / (/ is the identity tensor) in (2.4.9), we get divr(T) =divr(o-/) = crdivr/ + Vpcr.
(2.4.10)
Using the relation divpl = divp/ = KcN, we obtain the last two terms on the r.h.s. of (2.4.7). Conservation of energy in the singular surface F In order to obtain conservation of energy in F, we shall first derive the equation of conservation of energy in R followed by equations of conservation of energy in Rs and Ri- Subtracting the sum of energy equations for Rs and RL from the energy equation for R, we get the energy equation for F. The equation of conservation of energy in R is — / pedv -\- — RS^RL
r
eda
= —
q- nda -
dRsudRi^
q • Vdl, dv
+ / ( T L - rs)N -Vda-^- j eUdl, r
dr
(2.4.11)
58
Thermodynamical
and Metallurgical
Aspects
of Stefan
Problems
where e is the specific internal energy, G is the surface energy of F per unit area, q is the heat flux vector of the bulk, q is the tangential heat conduction vector in F, {TI — TS)N-V is the work done by the body forces in F and U = —\V\N • n \s the heat lost or gained due t o geometry of Rs U RL djs N may have a non-zero component along n. Note t h a t {TL — Ts)N appears as a body force in (2.4.7) and the tangential work done by this force is zero. The angle which N makes with n is called contact angle. It is still being debated on how t o prescribe the boundary conditions a t the contact point [54]. By calculating
Fig. 2.4.1. Geometry in generalized Stefan condition. the time derivative of the first term on t h e l.h.s. of (2.4.11) by first principles, it can be proved t h a t ^
J pedv= I
p^^+Jp{es-eL)V-Nda.
(2.4.12)
RS^RL
RS^RL
It may be noted t h a t the region R is time independent but Rs and RL vary with time in R and this point should be taken care in calculating the time derivative to get the last term in (2.4.12). On balancing the energy separately in Rs and RL^ we get des / P-Kfdv
=-
dcL / P-^dv = RL
qs • nda dRs
Rs
qs ' Nda,
(2.4.13)
qi • Nda.
(2.4.14)
r
I qi' nda + ORL
r
Adding (2.4.13) and (2.4.14) and subtracting the sum from (2.4.11), we obtain j p[es - eL)\V\da^
- j eda = - jj^qi r
- qs) • Nda -
j^q-udl Jdv
+ / ( T L - Ts)N • Vda - f e\V\N - ndl. (2.4.15) r ar
2.4 Non-material Singular Surface: Generalized Stefan Condition
59
The second term and the last term on the r.h.s in (2.4.15) should be converted to a surface integral over F to obtain an equation for local energy balance in F. The conversion of the second term to the surface integral is straight forward. In [51], the contribution of the last term on the r.h.s. of (2.4.15) has been included as the third term in (2.4.16). Local balance of energy in F gives p{es - eiWl
+ ^' - ^ceV • N + d i v r | + {QL - Qs) • N - {TL-TS)N
e = ^
+ V- grade.
• V - 0, (2.4.16)
(2.4.17)
With the help of (2.4.6), the last term in (2.4.16) can be rewritten as {TL - Ts)N • y = - ( divrf) • V.
(2.4.18)
Equation (2.4.16) is called a generalized Stefan condition. Let us now briefly describe surface tension and its relation to G. The effect of surface tension or capillarity can be explained on the basis of the assumption that on the outer surface of the liquid there exists a surface layer which could be a few molecules thick and whose properties differ from those of the bulk liquid. The surface film and the liquid can be considered as two phases of the liquid in equilibrium. The atoms in the free surface of a sohd or a liquid have higher free energy than those inside because they have no neighbours and hence no cohesive bonds, on one side. Therefore, in equilibrium, a tension develops on the surface of the liquid. This tension is related to pressure by a relation of the form (2.2.12). a is often assumed to be identical to the free energy of the surface. Prom the relation (2.1.15), we get a = e-TS,
(2.4.19)
where T is the phase-change temperature and S is the entropy. The Stefan condition (1.4.8) can be easily derived as a particular case of (2.4.16). For the derivation of Gibbs-Thomson effect from (2.4.16), see [51]. Some other observations made in [51] are as follows. A simple criterion for the instability of the interface can be derived using (2.4.16). Let G' = 0, G 7^ 0, ^ = 0 and f = al where the surface tension a is constant. Under these assumptions, the relation given below can be obtained.
|y| . }''-f}'^^.
(2.4.20)
Here, / is the latent heat per unit mass (for simplicity take Cs = CL)- Consider a very small sohd growth protruding into the liquid (see Fig. 2.4.2.) at the solid-hquid interface. In this case Kc is negative and if (G — a) > 0, then \V\ is increasing. This is an ideal situation for the dendritic growth as any solid protrusion in the liquid will grow. If (G -a) < 0, then there is retardation. Ulp+Kc{e-a) -^ 0, then \V\ -> 00 and dendrites
60
Thermodynamical and Metallurgical Aspects of Stefan Problems
will grow indicating instability of the solid-liquid interface. If lp-\- KC{E — a) -^ OO, then |17| - ^ 0 . This criterion may indicate branching or side protrusion in the dendrite. If the mean curvature is positive (see Fig. 2.4.3.) and G — a > 0, then the surface F is locally retarded and for G — cr < 0 it would be advanced. In the derivation of (2.4.6) and (2.4.16), both the sohd and liquid phases have been considered at rest. The motion of phases has been taken into account in [49] and a more general solidification theory has been presented which takes into account the coupling between thermal fields and kinematic fields. The liquid region is an ordinary Newtonian liquid and the solid phase an elastic material. The specific internal energy, specific entropy, stress tensor and the heat flux vector in the bulk are considered as functions of temperature, temperature gradient, deformation gradient and the rate of deformation gradient. In Clapeyron's equation (2.1.27), it is assumed that Gibbs free energies of hquid and solid phases (two different phases of the same substance) are the same and that the system is in thermodynamic equilibrium. This means that neither phase can grow or decrease. In [50], a new Clapeyron's equation has been derived, which holds in the presence of nonuniform fields of pressure, temperature, density and velocity, and which does not require the equality of Gibbs functions across the interface as long as their difference is a function of time only, or in particular a constant along the interface. It was shown that even in the presence of constant pressure along the interface, the interface temperature can be changed by increasing or decreasing the velocity at which the process of solidification is taking place. ////// //////////// /////////////// ' f f/ ff/ / / n / n f///////////////
//////////////////
f/ f / n / / / / / n / / f f
/////////////''/{> /////////////^i //////////////^ /////////////
n //^
/ / / / / / / / / / / L
///////////////////'
//////////
ftj.
/i.
//////////A
// /ff///f f//
f ////
/f uf n /n f f////
f t //
t
/ / n f (u uf / // n / / ( / / n / f / / / // f n / n / f\ t H f/f //////////f f f n // f f / / / n / /1 i f^ ///////////////O f/////////// U/J
' ; ' / S o l i d ; ; F - ^ N ; V - " Liquid //////////J /////////// //f////////\
//////////// ////////////> u / / n f n / / (/ /y f / / / / n / n / f / / /1 / / / / / / / /
Fig. 2.4.2. Mean curvature K^ is negative
f//f
f f f f f
fi
Fig. 2.4.3. Mean curvature Kc is positive.
Chapter 3 Extended Classical Formulations of n-phase Stefan Problems with n > 1 3.1 3.1.1
One-phase Problems An extended formulation of one-dimensional one-phase problem
One-phase Stefan problems are mainly of three types. In the first type, one of the phases is considered at the melting temperature so that there is temperature gradient present only in one of the phases. Such problems could arise either in melting or solidification. Problems of the second kind are ablation problems in which a solid is melting and the liquid drains out instantaneously. In the third type, problems have to do with solidification of supercooled liquids, in which the temperature of the liquid is less than the equilibrium temperature leading to some changes in the energy balance equation at the interface. The melting problems in which the solid is superheated also belong to this class of problems. One-dimensional one-phase problems with different formulations have been studied from the perspectives of mathematical analysis and computation. Instead of giving complete formulations of problems arising in different contexts, we give here a general formulation of the one-phase problem so that in later sections other one-phase problems can be treated as particular cases of this general problem. This will avoid unnecessary repetitions of some details common in different formulations. A fairly general one-phase Stefan problem with nonlinear parabolic equation and nonlinear free boundary conditions has been discussed in [55] and is being presented here. Consider a time interval (0, £*). For each t G (0,t=,), consider the set J2(i) of functions p(r) which are continuously differentiable in [0,t), continuous in [0,t], and are such that P{T) G (bo, hi) for r G (0,^), p(0) = ^ 6i > 6 > 6o > 0. For S{T) G EiQ 61
62
Extended Classical Formulations ofn-phase Stefan Problenis with n > 1
and t G (0, t^) define the set Q(t) = {{X,T) :0< x < S{T), 0 < T < t} and let C^^^{n{t)) be the set of functions T(x, t) which are continuous in Q{t) together with their x-first derivatives. Assume that a(x, t,?/o,?/i,Po,Pi) is a positive function f o r O < j : < o o , 0 < ^ < t*, —cxD < ?/o, yi < oo, po > 0, —cxo < pi < oo. The function q(x,t,yo,yi,po,pi) is defined in the same domain as the function a, and h(x), f{t) are defined for 0 < x < 6, 0 < t < i*, respectively, and ip{x,t) is defined for x > 0, 0 < t < t^. The problem is to find a triple {t^, S{t),T{x, t)) such that
(i) 0
I,
(ii) S{T) e E(Q. (iii) T{x,t) G C^'°(n(^=^)), Txx and Tt are continuous in ^(i*), (iv) the following equations are satisfied. Tt-a\x,t,T,T,,S,S)n,
= q{x,t,T,T,,S,S),
{x,t) G Q(^*),
(3.1.1)
T ( J : , 0 ) = /I(X),
a; G [0,6],
(3.1.2)
T{0,t) = f{t),
^G(0,^*),
(3.1.3)
T{S{t),t) = ^{S{t),t), S{t) = (l){S{t),t,T,{S{t),t)),
t G (0,^0, ^ G (0,^*).
(3.1.4) (3.1.5)
Here, (t){p,t,yi) is a function defined for p > 0, 0 < ^ < ^*, - c o < ^i < oo, a^ is the thermal difTusivity which has been considered as a function of independent and dependent variables, q is the heat source per unit time per unit volume. The temperature T{x,t) can be made dimensionless by dividing it by Tm so that at the free boundary T = 1. By redefining the temperature as T — 1, the temperature at the free boundary can be taken to be zero. In the literature, both 0 and 1 are used as dimensionless isotherm temperatures. If we make the transformation T = T — ip{x,t) in (3.1.1)—(3.1.5), then T becomes zero at the free boundary. A mere formulation of the problem as given in (3.1.1)—(3.1.5) does not guarantee that its solution exists. Even if the solution exists, T{x,t) and S{t) may not satisfy the regularity conditions mentioned above. The appropriate conditions to be satisfied for the existence, uniqueness, well posedness, etc., of the solution of (3.1.1)—(3.1.5) and solutions of some other Stefan problems will be discussed in Chapter 10. Presently we are concerned with the formulation. If the region 0 < x < S{t), S(0) — b is taken as solid (could be identified as ice) and the region x > S{t) {S{t) < x < bi) is taken as liquid at the equilibrium temperature
3.1 One-phase Problems
63
0 (or 1) (identified as water) and there is cooling at x = 0, then we have a one-phase solidification problem. The growth of the solid in the region x > b depends on some appropriate conditions which the prescribed data has to satisfy. In particular the data has to satisfy some compatibility conditions at x = b for S{t) to be positive. If 6 = ^o = 0 then the data has to satisfy some compatibility conditions at x = 0. If the region 0 < x < S{t) is liquid (could be identified as warm water) and the region x > S{t) is sohd (identified as ice) at the equilibrium temperature and if there is heating at x — 0 then we have a one-phase melting problem. In the melting problem also some compatibility conditions are to be satisfied by the data for S{t) to be positive. These compatibility conditions have been discussed in Chapter 10 for several problems. Any of the boundary conditions (A) discussed in § 1.4.4 can be prescribed at x = 0 instead of (3.1.3). Often the mathematical formulations of physical problems are presented in the form of a set of differential equations and boundary and initial conditions without defining the relevant function spaces in which a solution is sought, the spaces to which the known functions belong, ranges of dependent variables, and domains of independent variables. In a rigorous mathematical formulation all such details should be mentioned as done for problem (3.1.1)—(3.1.5). However, due to length constraints it is not possible to do so for every problem discussed in this volume. These details may differ for different problems. A physical problem can be generalized to any extent but it may not be possible to throw any light on of its solution. We shall discuss only those generalizations of Stefan problems which have been rigorously explored. When dealing with the classical formulation of a Stefan problem, its solution should satisfy some regularity conditions given ear Her in § 1.4.6 and as above. In the place of (3.1.4) and (3.1.5) more general boundary conditions of the type T{S{t), t) = Z [dT/dx, S{t), S{t)) ,
(3.1.6)
W{T, dT/dx, S{t), S{t)) = 0, on X = S{t),
(3.1.7)
and
can be prescribed. Here, Z and W could be functionals and need not be pointwise functions of their arguments (cf. § (3.3.33)).
3.1.2
Solidification of supercooled liquid
Consider the following problem: Tt = Tix, in A , = {(x, t):0<x<
S{t), 0 < t < t,, S{0) = 1},
T(x, 0) = To(x) < 0 , 0 < X < 1,
(3.1.8)
(3.1.9)
64
Extended Classical Formulations ofn-phase Stefan Problems with n> 1 Tx{0,t)=g{t)>0, T{S{t),t)
= 0,
T^{S{t),t) = -S{t),
0
(3.1.10) (3.1.11) (3.1.12)
By appropriate scaling of time and/or length, the various parameters have been taken to be unity in (3.1.8)—(3.1.12). This problem is concerned with the sohdification of a supercooled Uquid which initially occupies the region 0 < x < 1 and ice at T = 0 occupies the region 1 < x < oo. It can be argued that the region 0 < x < 1 cannot be solid. For if it were so then the region x > 1 would have to be liquid at the temperature zero and solidification would begin in the liquid. This would further imply that Tj:{S{t)^t) and S{t) are positive, violating (3.1.12). This problem will be referred as supercooled Stefan problem or in short SSP. The analysis of such problems has been presented in § 4.4.1.
3.1.3
Multi-dimensional one-phase problems
The formulation given in (3.1.1)—(3.1.5) can be extended to multi-dimensional onephase problems if the free boundary is defined by the equation ^{x^t) = 0, x ^ R^ as in §1.4.1. Multi-dimensional problems have not been investigated as exhaustively as one-dimensional problems and often the problems studied are not as general as described in the formulation (3.1.1)—(3.1.5). A typical multi-dimensional problem is the ablation problem described below. A three-dimensional ablation problem During melting of a solid, if the melt is removed as soon as it is formed, heat flux has to be prescribed at the phase-change interface for further melting to take place. Melting of a piece of ice when water formed is removed instantaneously and the melting of the surface of a spacecraft during re-entry into earth's atmosphere are examples of one-phase ablation problems. Consider a half space z > 0 which, at time t = 0 is in a solid state. Heat input Q{x,y,z,t) > 0 is prescribed at the free boundary z = S{x,y,t) = S(t) and it will be assumed that melting starts instantaneously at ^ = 0 (this is a minor assumption, see § (3.2.2)). The melt is drained out as soon as it is formed. The problem is to find the temperature T(x, y,z,t) and the phase-change boundary which we shall denote by a short notation as 2 = S(t). The following dimensionless equations are to be satisfied. ^
= kV^T , z > S{t) , t > 0; S{0) = 0,
(3.1.13)
T(.T,2/,2,0U = /(x,?/,2); /(a:,i/,0) = l,
(3.1.14)
3.1 One-phase Problems
65 T(x,y,z,t)l=s(t)
= l,
5(0) = 0,
(3.1.15)
The melting temperature has been taken to be unity. The energy balance at 2; = S{t) is given by the equation
Qi.,y,z,t)U.s + K{l^{-j
dsV
fdSV\ dT +^^j Kj
Ip'i-
(3.1.16)
A Signorini-type boundary condition Suppose in the above ablation problem melting starts at t = ^0 > 0, (instead of ^ = 0) where ^0 is unknown. Then we have two types of boundary conditions on 2: = S{t)^ i > 0. T(x,t,5(t),t)
r(x,?/,5(i),t) = l, | g + / f ^ |
=0, ^ = 0 ; t > 0 ,
(3.1.17)
= / p ^ , ^ > 0 , i>*o,
(3.1.18)
where n is the unit normal pointing into the solid. There is another way of expressing (3.1.17) and (3.1.18).
T{x,y,z,t)
{Q + KdTldn}^^sit)>^^
> t>0,
z = S{t)
{r(x, y, 2, t) - 1} {KdT/dn + Q} = 0. ^ riT
ri^
t>0; f >0
(3.1.20)
The flux prescribed condition at the free boundary is now an inequality and not an equation. Such boundary conditions are called Signorini-type boundary conditions [56]. When melting or solidification does not start at ^ = 0, then upto time t = ^0 > 0 or in other words till the time the temperature at the boundary z = 0 becomes the melting temperature, a pure heat conduction problem without phase-change occurs. At t = to, the temperature of the half space is to be calculated, which serves as the initial temperature for the phase-change problem. In essence we are solving two heat conduction problems. If a Signorini-type boundary condition is prescribed, then we are solving one problem but the boundary condition is specified in terms of an inequality. This type of formulation is more suitable for mathematical analysis of ablation problems and their numerical computations. It is possible to obtain variational inequality formulations for some problems with Signorini type boundary conditions (cf. [22]). In problem (3.1.13)—(3.1.16), the melting starts over the whole boundary z = 0. However, depending on the boundary conditions it is possible that the melting begins only over a portion of 2; = 0. Such problems will be discussed in § 3.3.2.
66
3.2
3.2.1
Extended Classical Formulations ofn-phase Stefan Problems with n > 1
Extended Classical Formulations of Two-phase Stefan Problems An extended formulation of the one-dimensionaJ two-phase problem
An extended formulation of problem (1.4.4)—(1.4.8) in the one-dimensional case and discussed in [57] is given below. Some results on the existence, uniqueness, and regularity of the solution to problem (3.2.1)—(3.2.10) will be presented in Chapter 10. V^'\T^'\x,t))
^Til^ - S^'^T^'^ = q^'\x,t),
D^^^ = {{x, t):0<x< T^'\x,0)
= h^'\x),
in D^'\
S{t), 0
0<x<
t^},
S{0), 0 < S{0) = b < I,
T(^^(0,t) = (t)^^\t), 0
mD^'\
D^^^ = {{x, t) : S{t) <x
T^'\S{t),t)
= T^'\S{t),t)
0
(3.2.4)
t,},
5(0) < X < 1,
T^^\l,t)=:(t)^^\t),
(3.2.2) (3.2.3)
= Tg^ - S^^^T}^^ = q^^\x,t),
T^'^\x,0) = h^^\x),
(3.2.1)
(3.2.5) (3.2.6)
0 < t < t*,
(3.2.7)
x^'\s{tu)n'\s{tu)-x^'\s{t\t)n'\s{tit) = S{t) + fi{S{t),t),
0
(3.2.8)
Here, S^^^ and J^^^ are inverse of thermal diffusivities. The boundary conditions (3.2.3) and (3.2.6) can be replaced by other boundary conditions such as T^'\0,t) = g^'\T^'\0,t),t),
0<^
T^^Hlt) = g^^\T^^\l,t),t),
0
Boundary conditions of mixed type can also be prescribed.
(3.2.9) (3.2.10)
3.2 Some Extended Classical Formulations of Two-phase Problems
67
The regularity conditions assumed in [57] for the analysis of the problem are: (1) 0(i)(^), (/)(2)(^) G C i [ 0 , q , (2) h'<^\x), h^^\x) belong to H\Q,l) where //^O, 1) is the Sobolev space [58] endowed with the norm ||/i||//i = ||^||L2 + ||^1|L2; h{x) = h^^\x) for 0 < X < 6 and h{x) = h^'^\x), 6 < x < 1, (3) the compatibility conditions for the initial and boundary data are satisfied, i.e., (j)^^\^) = h'<^^{0) and (/)(2)(1) = /i(2)(l). ^(i) and S^^^ are constants, (4) The functions q^^^ and q^'^^ are Holder continuous in D^^^ and D^'^\ respectively, with respect to x (or t) and \q^^^{x, t)\ < Q (constant), (x, t) G D^''\ z = 1, 2, (5) /(x, t) and f^ are continuous and bounded in D = D^^^UD^'^^ and /-^x — ft is bounded and locally Holder continuous in D with respect to x (or t). By redefining the temperature as {T — f{x, t)), (3.2.7) can be transformed to T^^^ = 0, z = 1,2. This transformation can be used without redefining the assumptions about the changed data and the coefficients in (3.2.1)—(3.2.8). This transformation is the physical motivation for the term iJ.{S{t),t) in (3.2.8). Some of these regularity conditions can be relaxed, see Chapters 10 and 11. The initial data and /(x, 0) should also satisfy some compatibility conditions given below. \h^^\x) - f{b, 0)1 < 71 (^ - a:)", 0 < X < 6,
(3.2.11)
\h^^\x) - f{b, 0)1 < 7i(6 - x)^, 6 < X < 1,
(3.2.12)
where 71 and a satisfy conditions 716^ > d, and 71(1 - 6)" > d. Here, d is such that \(t)^'^{t)\ < d, t>0. and
x^'\xi'\
(3.2.13)
x i l xf\ i = 1,2 are continuous in D
\x^'Kx,t)\ < 0, (x,t)eD,
(3.2.14)
for some suitable /3 > 0. If the boundary conditions are of the type (3.2.9) and (3.2.10), then g^^\ z = 1, 2 have to satisfy some conditions (cf. [59]). For given q^''\ (j)^'-\ h^^\ x^^\ ^ = 1)2, and/(x,t) and /i(x,^) satisfying the conditions imposed on each of them mentioned above; find t^, T^^^(x, t),T^'^\x, t) and S{t) such that equations (3.2.1)—(3.2.8) are satisfied. In principle, the boundary conditions (3.2.7) and (3.2.8) on X = S[t) can be replaced by conditions of the form T'^^\S{t), t) = T^^\S{t), t) = Zi [dT^'^/dx, Wi[T^^\T^''\dT^^^ldx,dT^^^ldx,
dT^''^/dx, S{t), S{t),) ,
S(i), S{t)) - 0, on X = S{t).
(3.2.15) (3.2.16)
Here, Zi and Wi could be functionals and need not be pointwise functions of their arguments (cf. (3.3.33)).
68
3.2.2
Extended Classical Formulations ofn-phase Stefan Problems with n > 1
Multi-dimensional Stefan problems of classes II and III
According to the change of phase initiating along the fixed boundary of the region originally occupied by the melt, classical Stefan problems can be divided into three classes [60]. In class I problems solidification or melting starts simultaneously at all points of a fixed surface of the region under consideration. In class II problems solidification begins at a 'portion' of the fixed surface. In class III problems it begins at a 'point' of the fixed surface. Class II and class III problems essentially differ from class I problems in that solidification (melting) in them both spreads along the surface and grows towards the interior. Under suitable assumptions, class I problems can be formulated as one-dimensional problems but class II and class III problems are necessarily multidimensional. The position of the interface in Fig. 1.4.2. is shown after solidification for a long time. It could be due to solidification at a point or over a portion of dGi{t) or over whole of dGi{t). After sufficiently long time, when the spread of solidification over dGi{t) is complete, then solidification growth will be only towards the interior. In Fig. 1.4.1, even for longer times, a class II problem persists. At least in the analytical solutions and the analysis of Stefan problems, this distinction between different classes affects both procedures and solutions. A formulation of a multi-dimensional classical Stefan problem has already been given in § 1.4. Some more formulations will be discussed later. Because of their complexity, class II and III problems have not been studied widely. They can be formulated as one-phase or two-phase problems. A three-dimensional one-phase ablation problem in a semi-infinite region with melting due to a 'hot spot' was considered in [61] and a shorttime asymptotic solution was obtained. We present below a class III two-phase Stefan problem discussed in [62] in which solidification initiates at a point due to a 'cold spot'. Consider an axisymmetric cylindrical problem in which R and Z are cylindrical polar coordinates. A superheated melt at time i = 0 occupies the region Q < RQ < R < oo, \Z\ < GO. This melt is being cooled and a known flux Qs{Z,t) which is symmetric in Z is prescribed on the surface R = RQ of the cylinder. The temperature /L(-R, Z) of the melt at t = 0 is a known quantity which is taken to be a symmetric function in Z and is such that hiR, Z)
= r„ , Z =
0,R=Ro
> Tm , Z ^ 0, Ro < R < oo,
1 1 = 0 , Z = 0,il = /?o; ^ ^ 0 ,
Z = 0,R=Ro.
(3.2.17)
(3.2.18)
The solidification will start instantaneously at the point Z = 0, R = RQ and with time it will spread along the surface R = RQ and also towards the interior R > RQ oi the cylinder. The equation of the solidification front can be written as R = S{Z,t)]
\Z\ 0.
(3.2.19)
3.2 Some Extended Classical Formulations of Two-phase Problems
69
\Z\ = b{t) is the equation of the spread of sohdification along R = RQ^ and R = S{Z, t) > RQ is the growth of the sohdified layer towards the interior of the cylinder. The dimensionless formulation of the problem is as follows: In the liquid region 2a
2
^
dv
=v
d^n
1 OTL
Qj.2
dr
d^n
S(z, y ) < r < oo, 1^1 < oo, y > 0, (3.2.20)
dZ^
(3.2.21)
Tiir, z, V)\v=o = hir. z), 1 < r < oo, 1^1 < oo, dTL dr
QL{Z,V).
\Z\>B{V),
y>o.
(3.2.22)
•-PQL{Z,V),
\Z\
y>0.
(3.2.24)
TUr, z, V)\r=sizy) = 1, \z\ < B{V),
V > 0,
(3.2.25)
Ts{r, z, V)\r=sizy) = 1, \z\ < B{V),
V > 0,
(3.2.26)
In the solid region
dTs dr
At the solid-liquid interface
1 + dz
2A ^99
dr
dr
r=S{z,V)
yg^,
\z\ < B{V),
V>0,
S{2,V)\v=o = I, B{V)\v=o = 0, 5(^,V)||,|=B(V) = 1.
(3.2.27)
(3.2.28)
The following dimensionless variables have been used in (3.2.20)—(3.2.28). z = Z/Ro, r = R/Ro, V = 2{kst/Riy''\
A = l/CsT^, P = KLIKS,
QL{Z,
a^ =
ks/h,
V) = QUz,t) • Ro/KgT^,
S{z, V) = S(Z, t)/Ro, Mr, z) = A(/f, Z ) / r „ , B{V) = b{t)/Ro.
(3.2.29)
70
Extended Classical Formulations ofn-phase Stefan Problems with n> 1
If the melt is superheated, i.e., if its temperature is greater than the melting temperature then for solidification to begin it is necessary (not sufficient), that the prescribed flux be infinite at ^ = 0. If at t = 0, / L ( 1 , 0 ) > 1, then we shall have a heat conduction problem (without phase-change) up to time t = tm > 0 such that at ^ = i^, conditions (3.2.17) and (3.2.18) are satisfied. Time can be rescaled and this instant of time can be taken as ^ = 0 with the initial temperature changed to the temperature at t = tm- A short-time analytical solution of a class II problem has been obtained in [63] in which solidification/melting starts at a portion of an edge in a plate. The prescribed flux need not be symmetrical. It is interesting to note that the growth of the free boundary for a short-time along r = 1 in the above cold spot problem could be of unusual type such as 0(t^/'') (similar results were obtained in [63]) but the growth the of free boundary along the interior is of well-known type such as of the order of t^^"^ or t or ^^/^. Although class II and III problems appear to be interesting and distinct from class I problems there are only some short-time analytical solutions to them in the literature. There is no report available on the existence, uniqueness and regularity of the solutions of these problems.
3.2.3
Classical Stefan problems with n-phases, n > 2
Some one-dimensional problems with more than two phases Classical Stefan problems with more than two phases are much more difficult to study than two-phase problems because of interactions among phases. Several types of generalizations of two-phase formulations to n-phase problems are possible. We shall report here some of the formulations studied in the literature. In principle, phase-change boundaries could be intersecting but such formulations have not be studied in detail. Some of these n-phase formulations are simple extensions of two-phase problems reported earlier in §§ 1.3 and 3.2.1. In an n-phase problem, it is difficult to prescribe the exact number of distinct phase-change boundaries without knowing the number of disjoint subregions and the physical situation. In several problems this has been taken to be (n — 1). In each phase a suitable parabolic heat equation is satisfied and at each phasechange boundary, two boundary conditions are prescribed (cf. § 1.3) The formulation of an n-phase one-dimensional problem in which the boundary conditions at the phase-change boundaries could be of type (1.3.5) and (1.3.6) has been considered in [64]. Odd and even numbered phases are identical so that there are only two different initial temperatures and two-different temperatures. The existence of a global solution to this problem which is nearly classical (the Stefan condition is satisfied in the integrated form) has been discussed in [64]. An analytical solution to a onedimensional n-phase solidification problem in the region 0 < x < oo has been discussed in [65]. At each free boundary, the temperature has a specified, constant, limiting value from each side. These limiting values may differ by a finite jump at different phase-change boundaries. Initially all the free boundaries coincide at x = 0 and the temperature takes the constant value Tm for x > 0 where Tm is the limiting value of the temperature as
3.2 Some Extended Classical Formulations of Two-phase Problems
71
x —> cxD. The Stefan condition is considered at the free boundaries. Densities of different phases could be different, giving rise to the movement of phases. Using local coordinates which are fixed in each phase, a similarity solution is presented. The formulation of the one-dimensional multi-phase problem reported in [21] is a little different. At time ^ = 0, an unbounded plate of thickness 2d and temperature To is dipped into a melt maintained at constant temperature Ti > TQ. It is assumed that a known constant heat flux q is maintained from within the melt to the plate and in the 'boundary layer' the temperature changes linearly in the direction normal to the surface of the plate. Let T^ be the freezing temperature of the melt and T„, the melting temperature of the plate. Depending on the magnitudes of different temperatures, the following three cases arise. Case I To < Tfc < Ti < Tn.
(3.2.30)
Since To < Tjt, crust forms on the plate till some time ^o > 0 after which the crust begins to melt as the temperature at the surface of the plate rises to Tk and at the free boundary heat flux q is acting. Melting will continue till time ^i > to > 0 when the crust has completely melted. After that, heating of the plate without a phase-change continues. The plate cannot melt because the melting temperature T^ of the plate is greater than Ti. For 0 < t < ti, there exists a two-phase problem with only one free boundary and for t > ti the problem is that of heat conduction in the plate without phase-change. Because of the assumption of symmetry it is sufficient to consider the region 0 < a; < oo. Case II To < Tfc < T„ < Ti.
(3.2.31)
At any given time there will be only one phase-change boundary. The freezing temperature of the melt is lower than the melting temperature of the plate. First the crust forms and then the crust starts melting and when the plate attains temperature T„, the plate starts melting. Case III To
(3.2.32)
There are three possibihties. (PI) The plate melts and the melt crystallizes instantaneously with the immersion of the plate in the melt. (P2) The melt crystallizes instantaneously but the melting of the plate starts late. (P3) The plate begins to melt instantaneously but the crystallization of the melt is delayed.
72
Extended
Classical Formulations
of n-phase Stefan Prohlenas with n > I
Problem ( P I ) corresponding to the possibility ( P I ) can be formulated as (not made dimensionless) follows. d^T dT a\x)--^ dx'^ = —^, dt 1 = 1,2,3, x ^ 5 i ( t ) , x^d] a\x)
x y^ 82(1), t > 0,
kl,
0 < X < S'2(t),
kl,
82(1) < x < d,
kj,
d<x
<
(3.2.33)
(3.2.34)
Si(t).
Here, x = Si{t) is the equation of the freezing front in the sohdification of the melt and x = S2{t) is the equation of the melting front in the melting of the plate. T h e subscripts 1, 2, and 3 refer to the regions 0 < x < 5*2(^), S2(t) < x < d and d < x < Si{t), respectively. = 0; Ti\j,^s2it) =Tn = T2\x=s2{t)', t > 0,
dx
=s,it) =n;
T2U=
Ki
dx
'-3\x=d+0]
K2^
-q -h K^-^
(3.2.36)
t> 0; Ti|,=o - To, 0 < x < d,
Ko
dT2 dx
= PiliS2(t),
K:
m dx
(3.2.35)
t > 0,
(3.2.37)
=d+0
for X = S2(t), t > 0; ^2(0) = d,
(3.2.38)
ox = pshSiit),
for X = Si{t),
t > 0; 5i(0) = d.
(3.2.39)
T h e motion of the hquid due to the difference in densities has been neglected. Problems (P2) and (P3) can be formulated on the same lines as Problem ( P I ) , but the time interval t > 0 has to be divided into several ones and in each interval an appropriate problem is to be formulated. For example in Problem (P2), if the melting of the plate begins at t = to then for 0 < t < to, we have a problem of pure heat conduction in the plate and crust formation in the melt. For to < t < ti, where ti is the time at which the temperature at X = d becomes Ti^, there wiU be both melting of the plate and freezing of the melt. For t > ti, crust starts melting and melting of the plate continues or plate might have completely melted by that time. At each stage, temperature and the position of the free boundary / boundaries in the previous stage are to be ascertained. In [21] the main interest in the study of Case III is to examine the possibilities of occurrences of three cases ( P I ) , (P2) and (P3). This requires short-time {t -^ 0)
3.2 Some Extended Classical Formulations of Two-phase Problems
73
analytical solution of Problem (PI) which was obtained by using fundamental solutions of the heat equation for a double layer and taking the limit as t ^ 0. Both necessary and sufficient conditions for the occurrence of the case (PI) have been obtained in [21]. The solution to Problem (P3) will exist if Si{t) < d ioT t > 0 and therefore Problem (P3) does not have a solution. It has also been shown in [21] that the conditions under which both (PI) and (P2) are possible cannot coincide. A three-phase problem with two free boundaries has been discussed in [66] which is concerned with both melting and evaporation. Consider a solid occupying the region 0 < a; < a (one-dimensional problem). The boundary x = 0 is insulated and the solid is heated at x = a. First the solid melts at x = a and for some time, solid and liquid regions separated by a free boundary occupy the region 0 < x < a. With further heating, when vaporization temperature is attained at x = a, the liquid starts evaporating. There will now be two phase-change boundaries, viz., liquid-vapour and solid-hquid. If the solid is enclosed in a container, then heat transfer in the vapour is to be considered and there will be three-phases and two free boundaries. If the vapour is allowed to escape, then there are two phases and two free boundaries. Using finite-difference and finite element methods, numerical solution of this problem has been obtained out by several workers
(cf. [66]). 3.2.4
Solidification with transition t e m p e r a t u r e range
A very pure metal has a fixed melting temperature, which is also its freezing temperature. In the case of alloys or metals with impurities, melting and freezing temperatures are not the same and phase-change takes place over a temperature range. Let freezing and melting temperatures be denoted by Ti and T2, respectively. For metals with impurities, both heat and mass transfer should be considered but if the concentration of impurity is small, then only heat transfer can be considered with phase-change taking place over a temperature range Ti < T < T2. The region whose temperature lies between Ti and T2 is called a mushy region. There are two phase-change boundaries. The solid-mush boundary separates the solid region from the mushy region and the liquid-mush boundary separates the liquid region from the mushy region. In [67] an analytical solution of a one-dimensional solidification problem in cylindrical symmetry with an extended freezing temperature range has been obtained. The finite-difference numerical solution of a onedimensional solidification problem in a finite slab with an extended freezing temperature range has been presented in [68]. We present below a two-dimensional formulation of an extended freezing temperature range problem whose finite-difference numerical solution is reported in [69]. and Y are dimensionless A two-dimensional region 0 < X < 1 , —a T2, the material is in a stable liquid phase and for Ti < T < T2, a mushy region exists. Cooling is done at the boundary of the rectangular region in such a way that the mushy region is sandwiched between stable
Extended Classical Formulations ofn-phase Stefan Problems with n > 1
74
solid and stable liquid regions. It will be assumed that the solid-mush boundary can be expressed in the form X — i?i(y, €) and the liquid-mush boundary can be expressed in the form X — R2{Y,t). Without loss of generality it can be assumed that the solidification starts at X = 0 at time t = 0. Even if there is some delay in the starting of solidification, for example, if the solidification starts at ^ = ^o > 0, then during the time 0 < t < to, there exists only a heat conduction problem without phase change whose numerical solution is generally considered to be simple and so assumed to be known by using well known methods. By redefining the time scale it can be assumed that the solidification starts at t = 0. The mathematical formulation of this three-phase problem in the dimensionless form as considered in [69] is as follows: In the solid region
^
" ""^ ( S ^ ^ 5 ^ ) ' 0 < ^ < RiiY.t). -a < y < a; i > 0,
= Fp{Y,t)oT
dx
m
Ts\x^o = Tj,{Y,t); i > 0,
(3.2.40)
(3.2.41)
: 0 , 0 < X < Ri{a,t),
t>0,
(3.2.42)
= 0, 0 < X < Ri{-a,t),
t >0,
(3.2.43)
dv dTs dY In the mushy region
(3.2.44)
dY
dY
0, Ri{a,t) <X
t > 0,
(3.2.45)
Y=a
0 , H i ( - a , t) <X < R2{-a, t), t > 0,
TM|t=o = g{X, Y)
(3.2.46)
(3.2.47)
3.2 Some Extended Classical Formulations of Two-phase Problems
75
In the liquid region
t - . ( g + g|.W,.)<;^<M>0. TLU^O - 9{X, y)>Tmi.
dX
OTL
dY
X>
(3.2.49)
R2{Y, 0),
0, ^ > 0,
0, R2{a,t) <X
(3.2.48)
(3.2.50)
0,
(3.2.51)
Y=a
STL
= 0, R2{-a,t)
dV
< X < 1, ^ > 0.
(3.2.52)
1.0,
(3.2.53)
At the solid-mush boundary X — i?i(y, ^) TS = TM =
(dR,y[jdTs
^OTM)
,,dRi
(3.2.54)
At the liquid-mush boundary X = R2{Y^t) (3.2.55)
1 +
dR2 dY
dR2 = Xd' ' dt '
g{R2{Y,0),Y)
= Tmi
(3.2.56)
(3.2.57)
In the above formulation, di and d2 are solid fractions present at solid-mush and liquidmush boundaries, respectively; fs is the soHd fraction in the mushy region, g{X, Y) is the initial temperature/Ti, b is the length of the plate, a is the breadth of the plate/6, T is the temperature/Ti, Tmi is the temperature of the liquid-mush boundary/Ti. Other parameters are defined below. a = thermal diffusivity. ^^/6^ A = KM I Kg, P2 = KL/KSA
= plb^ / {K stmTi), (3.2.58)
76
Extended Classical Formulations ofn-phase Stefan Problems with n > 1
tm is the time taken for the hquid to attain the temperature Ti ai X = 0. Densities of all the three regions have been taken to be equal and are denoted by p. The constant di in (3.2.54) is generally taken as unity. But in the case of 'eutectics' it can be taken to be less than unity. At the liquid-mush boundary, c/2 — 0. If in (3.2.56), 6/2 — 0 is taken then we have an implicit free boundary condition which is not convenient for the numerical solution. By taking d2 very small, an explicit free boundary condition can be generated. In § 3.3 some transformations to convert an implicit free boundary condition to an exphcit free boundary condition are given but this treatment may give rise to some other difficulties in the numerical schemes. Although it is not a rigorous mathematical convergence proof, a reasonably accurate solution can be obtained if the numerical results converge as ^2 is gradually decreased. The accuracy of the numerical solution can be further checked by some methods such as integral heat balance calculation (cf. [69]). The solid fraction in the mush depends on various physical parameters, such as the temperature of the mush, and the width of the mushy region. An exact mathematical expression for the solid fraction in the mush cannot be given and in its absence some approximate mathematical models are proposed. Two such models have been considered in [69] for the numerical solution of the problem. Model I
fs{x,y, t) = {d,{Tmi - TM{X, y, t)) - d2[\ - TM{X, y, t))}/(Tw - i.o). (3.2.59) Model II fs{X,Y,t)
= [d,{R2{Y,t) - X} - d2{Ri{Y,t) - X}]/{R2{Y,t)
- Ri{Y,t)}.
(3.2.60)
In the first model, fs is a linear function of the temperature of the mush and in the second model fs is a linear function of the 'width' of the mushy region. Thermodynamically, the heat extracted from the system at the fixed boundaries should be equal to the heat given out by the system during sohdification. If this balancing of heat is done in an integrated way over the whole region and over a period of time, then it is called integral heat balance verification. Integral heat balance is satisfied for model I but not for model II. In the first model we can calculate fs from the temperature (calculated temperature) of the mush and this procedure is thermodynamically consistent but in the second model corresponding to the calculated fs, the temperature is calculated in the numerical scheme and therefore integral heat balance is not satisfied.
3.3
Stefan Problems with Implicit Free Boundary Conditions
If in the place of (3.1.12), we have the condition dT dx
-0, x-=S{t)
(3.3.1)
3.3 Stefan Problems with Implicit Free Boundary Conditions
77
then velocity of the free boundary is not exphcitly prescribed. As stated earUer, free boundary conditions of the above type in which S{t) is not prescribed exphcitly are known as implicit free boundary conditions. Free boundary condition (3.1.12) is an explicit boundary condition as S{t) is prescribed in it. Our main aim in this section is to present some transformations which convert a Stefan problem with an implicit free boundary condition to a Stefan problem with an explicit free boundary condition. An extensively studied problem with an implicit free boundary condition known as oxygendiffusion problem will also be discussed later in detail.
3.3.1
Schatz transformations and implicit free boundary conditions
Problem 3.3.1. Find t^, T{x,t) and S(t) satisfying the following system of dimensionless equations T:,:,-Tt = F{x,t),
a<x<
aT:,{a,t)-\-/3T{a,t)
S{t),0
= f{t),
0 < t < t,,
T{x,0) = (pix), -oo
= 5(0),
T{S{t), t) = g{S{t), t),0
t) + h{S(t), 0; 7 7^ 0, 0 < t < t,.
(3.3.2) (3.3.3) (3.3.4) (3.3.5) (3.3.6)
All the thermophysical parameters in this one-phase problem have been taken to be unity. This is possible by suitably choosing time and/ or length scales. In those Stefan problems in which the effect of thermophysical parameters is not to be investigated, it is convenient to take parameters to be unity, a and p are constants. Depending on the nature of the data, the problem (3.3.2)—(3.3.6) could be either a melting problem or a solidification problem. We report here some of the assumptions and for complete details see [70]. (i) F(x, t), g{x, t), h{x, t) and F^ G C for a < a; < oo, 0 < ^ < t*.
(ii) f{t) e C, 0 < ^ < t*. (iii) If a < 6, then (/> G C^ for a < x < 6 and 0(6) = .^(6,0), 0'(6) = h{b,0). These are compatibility conditions to be satisfied by the initial temperature and the functions g and h at the free boundary. (iv) In addition to the regularity conditions to be satisfied by S{t) and T(x, t) in the classical solution of a Stefan problem mentioned in § 1.4.6 it will be assumed that
78
Extended Classical Formulations ofn-phase Stefan Problems with n > 1
Txxx, Txt ^ C ioT a < X < S{t), 0 < t < t^. The significance of this assumption wih be made clear later. Problem 3.3.2. Consider problem (3.3.2)-(3.3.6) with some changes. Let j{S{t), t) = 0 in (3.3.6), so that T,{S{t),t)
= h{S{t),t).
(3.3.7)
Make some additional assumptions given below. gx{x,t) ^h[x,t),
a < X < oo, 0 < t < t*,
(3.3.8)
and a = 1, /3 = 0. (3.3.9) Problem 3.3.3. Let ^{x,t) = g{x,t) = h{x,t) = 0 and a = 0, /3 = 1 in Problem 3.3.1. In this case the free boundary conditions are given by T{S{t),t) = 0, 0
(3.3.10)
Tx{S{t),t) = 0, 0
(3.3.11)
The equivalence of Problems 3.3.1.—3.3.3. is established by the following propositions. Proposition 3.3.1. If {S,T) is the solution of Problem 3.3.2., then (5, f) where V = Tx and S is the same as in (5, T), is the solution of the following Stefan problem. Vxx -vt = F^(x, t), 0 < X < S{t), 0
0
v{x, 0) = (/)'(x), a<x
(3.3.12) (3.3.13)
= 5(0), 0
+ F{S{t),t) + gt{S{t),t), 0
(3.3.14) (3.3.15) (3.3.16)
Equation (3.3.12) can be easily derived if the partial derivative of (3.3.2) with respect to X is taken. If the material time derivative of (3.3.5) is taken, then we obtain n{S(t),
t)S(t) + Tt = gx{S{t), t)S + gt.
(3.3.17)
On using (3.3.7) and (3.3.2) in (3.3.17), the equation (3.3.16) can be obtained. Equation (3.3.17) suggests that we impose the condition g e C^, b < x < oo. Other conditions in (3.3.13)—(3.3.15) can be easily derived.
3.3 Stefan Problems with Implicit Free Boundary Conditions
79
Proposition 3.3.2. Conversely, if the pair (-S, v) is the solution of (3.3.12)—(3.3.16) then the pair {S, T) where S is the same as in [S, v) and T(x, t) is defined by S{t)
T{x, t) = [v{p, t)dp + g{S{t), t), a<x<
S{t), 0
(3.3.18)
X
is the solution of Problem 3.3.2. On repeated differentiations of (3.3.18) with respect to x, we get T, = v{x,t) and T.^, - v^.
(3.3.19)
Differentiation of (3.3.18), with respect to time gives S{t)
Tt = - J vt{p, t)dp - v{S{t), t)S{t) + g.{S{t),t)S + gt{S{t), t).
(3.3.20)
X
Also S{t)
S{t)
j vt{p, t)dp = j (v,, - F:,)dp = V:,{S{t), t) - V:,{x, t) - F{S{t),t) + F{x, t). X
(3.3.21)
X
In order to obtain (3.3.2) from the solution of (3.3.12)-(3.3.16), we use (3.3.21) in (3.3.20) and then substitute v^ from (3.3.16). Derivation of other conditions in Problem 3.3.1. is straightforward. Even if a = 0 and /3 = 1 in (3.3.3) the substitution f = T^ works. Differentiation of (3.3.3) {a = 0) with respect to t gives Tt{a, t) = T,,(a, t) - F(a, t) = f'{t) or ^;,(a, t) = f{t) + F(a, t).
(3.3.22)
In this case it has to be assumed that Tt and T^x are continuous at x = a and f{t) G C^. We conclude that Problem 3.3.1. and the problem defined by equations (3.3.12)—(3.3.16) are equivalent. Equation (3.3.16) is an explicit free boundary condition. Proposition 3.3.3. If (5,T) is the solution of Problem 3.3.3., then (S^v), where V — Tt, is the solution of the following problem. Vxx -vt = Ft{x, t), 0<x v{a,t) = f{t),
< S{t), 0
v{x, 0) = (t)"{x) - F{x, 0), a<x
(3.3.24) = 5(0),
v(S{t),t) = 0, 0
(3.3.23)
(3.3.25) (3.3.26)
t,.
(3.3.27)
80
Extended Classical Formulations ofn-phase Stefan Problems with n > 1
Equations (3.3.23)—(3.3.25) suggest that we impose the conditions that Ft and f'{t) G C and (t){x) e C^. The derivation of equations (3.3.23)—(3.3.26) is simple. To obtain (3.3.27), differentiate (3.3.11) with respect to t and use (3.3.2). Proposition 3.3.4. Conversely, if {S,v) is the solution of (3.3.23)-(3.3.27), then (5, T) where S is the same as in {S, v), S is of one sign, and T(x, t) is given by S{t) S{t)
T(x, t)=- f X
f {v{p, t) + F(p, t)]dpdi, a<x<
S{t), 0 < t < t,,
(3.3.28)
T]
is a solution of Problem 3.3.3. To derive (3.3.2), differentiate (3.3.28) twice with respect to x so that S{t)
dT r — = - y {v{p, t) + F(p, t)}dp
(3.3.29)
X
and d'^T dT — = v{x, t) + F(x, t) = -^ + F[x, t). (3.3.30) On substituting x = S(t) in (3.3.28) and (3.3.29), we obtain (3.3.10) and (3.3.11). If it is assumed that /(O) = 0(a) and 0(6) = 0 then (3.3.3) and (3.3.4) can be obtained. Although our main concern in this section is to show how the transformations v = Tj. and v = Tt transform Stefan problems with implicit free boundary conditions to Stefan problems with explicit free boundary conditions, some remarks on the analysis of Problems 3.3.1.—3.3.3. will be in order (see also Chapter 10). If T^ is continuous in a <x < S{t), then the relation S{t) = —Tj:{S{t),t) (for simplicity take 7 = 1 and h = 0 in (3.3.6)) implies that S{t) is continuous. This is true for the more general boundary condition (3.3.16) also if continuity assumptions are made for other functions involved. If the free boundary condition is of the form (3.3.7) or (3.3.11), then even if T^; is continuous, it cannot be directly concluded that S{t) is continuous. Existence of unique solutions of Problems 3.3.2. and 3.3.3. has been discussed in [70] under suitable data assumptions. If a < 6 and the data satisfy appropriate assumptions, then it can be proved that 0 < S{t) < A, 0 < t < to < oo ior some constant A (see Proposition 10.1.20.). The method of proof is the same as in [71]. In the place of (3.3.2) a quasi-linear heat equation with some constraints can also be considered. Conversion of a Stefan-type problem to a Stefan problem We consider a one-dimensional Stefan-type problem with phases 1 and 2 in which phase 1 occupies the region 0 < x < S{t) and phase 2 occupies the region S{t) < x < 1. This problem differs from a Stefan problem only in the free boundary conditions. Let T^^^
3.3 Stefan Problems with Implicit Free Boundary Conditions
81
and T^^^ be temperatures of phases 1 and 2, respectively. Tj^^ and TJ^^ are prescribed on X — S{t) and another boundary condition on S{t) is given in the form of a relation S = f{T^'\S{t),t),
T^^\S{t),t)).
(3.3.31)
If the transformations v^^^ — Tj^^ and f ^^^ = T^'' are used in the Stefan-type problem (equation (3.3.31) is not a Stefan condition) mentioned above and the problem is formulated in terms of v^^\x,t) and v^'^\x,t), then at the free boundary v^^^ and f^^^ will be prescribed. If the heat equations in the two phases are of the form (3.3.2), then t
t
T « ( 5 ( i ) , t) = J v^\S{r),r)dr 0
- j F^'^iSir), r)dT, i = l,2.
(3.3.32)
0
In view of (3.3.32), the equation (3.3.31) can be written as S = Pt{v^J\v^^\S{t)),
(3.3.33)
where for any t G [0,t^], Pt is a functional (a real valued function) acting on functions S{t), vi^^x.t), v^^\x,t), X e [0,S{t)] in vi^\x,t) and x G [S{t),l] in v^^\x,t) and 0 < t < t^ < oo. As explained earher, the Stefan condition (3.3.33) has been considered in the functional form and not as a heat balance condition.
3.3.2
Unconstrained and constrained oxygen-diffusion problem (ODP)
We shall first discuss a one-dimensional oxygen-diffusion problem, and use a shorter notation ODP for it. This problem was first formulated in [72] and studied later by several authors from various view points such as the existence and uniqueness, analytical and numerical solutions. Oxygen is fed to a tissue at the boundary a: = 0 at which a constant concentration CQ of oxygen is maintained. It is assumed that oxygen diffuses through the tissue and is absorbed at a constant rate a per unit volume. After some time, a steady state is reached. Suppose that in the steady state oxygen has penetrated upto a distance XQ in the tissue. Then, at x = XQ, both the oxygen concentration and the flux are zero. Steady-state concentration and unknown XQ can be easily obtained. If suitable dimensionalization is carried out as in [72], then the steady-state concentration can be obtained as 0.5(1 — x)^, 0 < x < 1. Once the steady state is reached, the boundary X = 0 is sealed. Oxygen diffusion and absorption starts again and the penetration depth of oxygen starts receding giving rise to a free boundary problem whose dimensionless formulation is given below. dc dt
d'^c 2 dx
1, m Du = {{x,t):0<x<S[t),^
(3.3.34)
c(x, 0) = 0.5(1 - x)^ 0 < X < 1,
(3.3.35)
82
Extended Classical Formulations ofn-phase Stefan Problems with n > 1 dc_ dx
= 0,
c{S{t),t) = 0, 0 < f
(3.3.36)
(3.3.37)
(3.3.38)
Here, c(x,t) is the concentration and x — S{t) is the free boundary. It may be noted that this problem is similar to Problem 3.3.3. and with the help of the transformation T = Q, it can be converted to a Stefan problem with an explicit free boundary condition. A typical feature of this problem is the non-compatibility of the initial and boundary data at a: = 0. Since concentration is a physical quantity, c{x,t) should be greater than or equal to zero. It can be proved rigorously that the classical solution (c, 5") of (3.3.34)-(3.3.38) exists with c{x,t) > 0, 0 < x < S{t), 0 < t < t^_ If the substitution T = Cx{x,t) is used in (3.3.34)—(3.3.38), then by using the maximum principle [9] it can be easily concluded that the maximum value of T(x, t) is zero and therefore Cx{x, t) < 0 for 0 < X < S{t). This implies c > 0 for 0 < a: < S{t) or c > 0 for 0 < x < S{t). We shall discuss the existence of the solution of ODP a little later. It may be pointed out here that ODP is closely related to the supercooled Stefan problem (SSP) formulated in (3.1.8)-(3.1.12). It can be easily checked that if the transformation T = Q is used in ODP then SSP formulation (see § 3.1.2) is obtained. If the concentration is defined by the relation X
c{x,t)=
I
C
f {T{r},t) + l)dr}d^,
(3.3.39)
S{t) S{t)
then ODP can also be obtained from SSP. Because of this connection between ODP and SSP, ODP can be regarded as a Stefan problem with an implicit free boundary condition even though ODP is concerned with the diffusion of mass and not with the heat transfer. The existence and uniqueness of ODP has been discussed in [73, 74, 75]. In [73], the existence and uniqueness of the solution of ODP was in essence proved by extending the domain of c(x, t) to the interval 0 < a: < 1 and taking c = 0 for S{t) < x < 1. The solution of this extended domain problem was obtained as the limit of a one-parameter family of problems. If a constraint c > 0 is added in the extended domain ODP formulation in 0 < X < 1, then it can be identified with an obstacle problem (see Chapter 7) whose variational inequality formulation exists. The existence and uniqueness of this obstacle problem has been proved in [74]. It has been proved that the obstacle problem never exhibits blow-up, in the sense that either (i) c(x, t) > 0, c(x, ^) ^ 0 for alH > 0 or, (ii) c(x,t) = 0 for t greater than some finite time t (extinction time). In [75], the existence and uniqueness was proved using fixed point arguments. It may be noted that if the initial concentration is given by (3.3.35), then it is not necessary to add the constraint c > 0 to the problem formulation of ODP as in this case it has been proved that the unique solution exists and c > 0.
3.3 Stefan Problems with Implicit Free Boundary Conditions
83
In the place of (3.3.35), let us take the initial concentration as c(x, 0) = g{x), 0 < X < 1, g^O.
(3.3.40)
We shall still call this changed problem as ODP. If it is assumed that g{x) is non-negative, non-increasing, sufficiently regular such as ^ G C^, g" is Holder-continuous at x = 1 and g{x) satisfies compatibility conditions g'{0) = 0, g{l) = g'{l) = 0, g"{l) = 1, then ODP with (3.3.40) in the place of (3.3.35) possesses a very smooth solution with c > 0 (cf. [75]). It is interesting to note that if g{x) = 0.5(1 — x)^ then g'{0) ^ 0 but still the existence and uniqueness of ODP can be established [75]. Constrained and unconstrained ODP We shall now consider ODP with initial concentration g{x) where g{x) can have any sign for x G [0,1]. Add the constraint c > 0 in the formulation. This problem will be called constrained oxygen diffusion problem or in short CODP. If the constraint c > 0 is not imposed in the formulation, then we have an unconstrained ODP or in short UODP. CODP is equivalent to solving the following equation with a suitable initial and boundary data. dc
H,[c),
~di~ Hy
(3.3.41)
is the Heaviside function. H,{c)
=
1,
o O
=
0,
c<0.
(3.3.42) The solution of CODP and UODP coincide as long as c > 0. The solution of CODP generally exists, is unique, and remains well behaved even for a sufficiently long time. The global behaviour of UODP is a delicate matter as a blow-up may occur in which S becomes infinite in finite time. Blow-up will be dealt with further in § 4.4. The solution T(x, t) of SSP is equal to Ct where c(x, t) is the solution of UODP but in general T(x, t) is not equal to Ct where c{x, t) is the solution of CODP. The non-consideration of the negativity set of c{x, t) in CODP deprives its solution of many interesting features which the solutions of UODP have, for example, the approach of the free boundary to a negativity set [75]. ODP in a radially symmetric domain. The existence of the solution of ODP in a cylindrical domain TQ < r < ri was considered in [76]. ODP in cylindrical symmetry can be easily formulated and this formulation is not being given here. The steady-state concentration Co(r) in cylindrical symmetry can be obtained as 2
2
2
co(r) = ^ ^ ^ - ^ I n ^ ,
(3.3.43)
84
Extended Classical Formulations ofn-phase Stefan Problems with n > 1
where Vi solves the equation i z i l i _ d i n l ^ = c , ci = co(ro). 4
^
(3.3.44)
fi
Here, Ci and ro are given, ro is the inner radius of the cylinder. A unique root ri > TQ of (3.3.44) can be obtained. The existence of a global solution has been proved by extending the existence of the local-in-time solution. The proof of the results in the cylindrical geometry are not on the same lines as proofs developed for ODP given in (3.3.34)—(3.3.38). It has been proved in [76] that under suitable assumptions there exists a t^ such that S{t^) = TQ. Quasi-static two-dimensional ODP and the Hele-Shaw problem A link between quasi-static two-dimensional ODP and the Hele-Shaw problem has been discussed in [77]. Let p(x, y, t), where t is a, parameter, be the pressure in the wellposed Hele-Shaw problem in which the free boundary is blown outward. The pressure in the liquid satisfies the equation V^j9 = 0 in Q C i?^
(3.3.45)
Let dQ, the boundary of the simply connected region Q, be a free boundary. On (90, we have dp p = 0, ^ = —Vn or Vp • Vw = —1, on ^ — w{x, y),
(3.3.46)
where t = w[x^y) is the equation of the free boundary and n is the unit outward normal to the free boundary. Equation (3.3.45) can be obtained by taking V = —gradp in the equation of continuity of an incompressible fluid whose flow is considered in a narrow channel [78, 79]. Free boundary is the surface of an expanding fluid blob. The continuity of pressure at the free boundary after appropriate scahng gives p = 0. The steady-state diffusion equation for concentration and the boundary conditions at the free boundary in a two-dimensional ODP can be written as follows. V^c - 1, in Q c i?^
(3.3.47)
dc c= -— = 0 on t = w{x, y).
(3.3.48)
The Hele-Shaw problem (3.3.45)—(3.3.46) can be converted to a steady-state twodimensional COD described in (3.3.47)—(3.3.48) by using the transformation given in (6.2.25). The problem defined in (3.3.47)—(3.3.48) is generally well-posed and so is the HeleShaw problem in which the boundary blows outward [77]. However the Hele-Shaw 'suction problem' is an ill-posed problem and the quasi-static ODP has recently been found to be a very eff"ective tool for revealing some unexpected regularity properties of the free boundary in the solution of unstable Hele-Shaw suction problem [77].
Chapter 4 Stefan Problem with Supercooling: Classical Formulation and Analysis 4.1
Introduction
In §§ 2.2.2 and 2.2.3, supercooling in the melt was taken into account by redefining the equilibrium phase-change temperature with the help of Gibbs-Thomson relation and the kinetic condition. When both these effects are considered together, we shall call the resulting equilibrium temperature relation as modified Gibbs-Thorns on relation. In this case, the new equilibrium temperature denoted by Tg is given by Te(x, t) = Tm- {(JI[S]E)K,
- aa\Vr,\l[S]E .
(4.1.1)
The symbols T^, a, 5*, Kc and V^ have the same meaning as explained in the earlier sections; the parameter a is related to the microscopic relaxation time, [*§]£;= difference in the entropies per unit volume of the liquid and the solid at the equilibrium temperature. The last two terms in (4.1.1) are the correction terms arising due to the curvature and the velocity of the interface, respectively. Equation (4.2.11) deals with the units associated with (J, S and a. Has the solidification of a supercooled liquid been correctly and satisfactorily modelled by the equilibrium temperature in equation (4.1.1)? To answer this question we have to go back to the formation of solid from a supercooled liquid in the solidification process. It may be noted that (4.1.1) was discussed earlier also with the help of some thermodynamical and metallurgical considerations. A rigorous mathematical justification will be provided now. The origin of solidification of a supercooled liquid of a pure substance is not in the growth of a sharp interface separating solid and liquid phases but in the finite size effect of the interface lying between stable solid and stable liquid phases as shown in Fig. 4.1.1. In other words, it can be said that the sohdification of a supercooled liquid cannot progress without the formation of a mushy region in front of the solidification front. The mushy region contains both solid and liquid phases and solid could be present in it in 85
86
Stefan Problem with Supercooling: Classical Formulation and Analysis
the form of dendrites of different types. Therefore it is clear that any mathematical modelling of the sohdification of a supercooled liquid should take into account solid, liquid and mushy regions, i.e., the finite-size effect of the interface. Does it mean that the classical formulation of the Stefan problem with supercoohng accounted for by the Gibbs -Thomson law is not justified as there is no mushy region in the formulation? The rigorous mathematical justification of the sharp freezing front model (classical formulation) with equilibrium temperature given by (4.1.1) comes from the fact that the modified GibbsThomson relation can be obtained if appropriate hmits of the phase-field model which considers the finite width of the interface, are taken. Some phase-field models are being discussed here with this objective. What should be the starting point in the modelling of solidification of a supercooled liquid? Research workers have used different approaches giving rise to different models. The differences arose due to the incapability of some models to predict accurately some physical situations or non-accept ability of some models due to the so-called inconsistencies. The existing literature on supercooling is vast. Some selected approaches in the modelling of solidification of a supercooled liquid are being presented here. It is not possible to describe here even these selected models completely. What is presented below is a suitable compromise between length restrictions and understability of these models.
Fig. 4.1.1. Finite size of the interface
4.2
A Phase-field Model for Solidification using LandauGinzburg Free Energy Functional
If the interface in the solidification of a supercooled liquid is assumed to be of finitewidth, then the modified Gibbs-Thomson relation would imply that there is a sharp interface which is continuously progressing through the finite-width interface. This in turn suggests exploring the enthalpy formulation given below in (4.2.1) in which a phase function 0(x, t) has been introduced which is continuously varying through the finite-
4.2 A phase-field model using Landau-Ginzhurg free energy functional
87
width interface. Enthalpy H{T) (per unit volume) is defined by H{T)
-
PsCsTs + ps^-cl>. T < 0 , 0 = - 1 ,
=
[-psl/2,
=
PLCLTL + PL^-(I>. T > 0 , 0 = 1 .
PLI/21
T = 0, - ! < ( / ) < 1, (4.2.1)
Here, (/> is called a phase function or an order parameter which in the classical formulation has the value — 1 in the liquid phase and 1 in the solid phase. Some authors take 0 = 1 and 0 = 0 in liquid and solid phases, respectively, and take / in the place of 1/2. The phase function (j) defines the state of the system. The phase-change temperature has been taken to be zero. H{T) vs T has been plotted in Fig. 4.2.1. Enthalpy is a multi-valued function of temperature but the temperature is a continuous function of enthalpy. The equation for energy in terms of enthalpy is given by ^
= V\KT),
xencR',0
(4.2.2)
Equation (4.2.2) is valid only in the distributional sense as at T = 0 the derivative of H is not defined (see § 5.2). K can have different values in different phases. It is clear from (4.2.1) that a sharp interface cannot be identified by T = 0 as done in the classical formulation because at T = 0 there exists a region and not a sharp interface. However a sharp interface can be identified by 0 = 0 as 0 is a continuous variable and 0 = 0 corresponds to the center of the mushy region. An equation for 0(x, t) is needed. If 0 is considered to be a continuous function of x and t, then it will look as in Fig. 4.2.2. When 0 = 0, T is also zero. T = 0 does not imply a sharp interface in the enthalpy model but 0 = 0 does. The starting point of a phase-field continuum model to be discussed below for the solid-liquid phase-change process has been taken here as the so-called Landau-Ginzburg free energy functional F defined as [80] nHx)}
= / { f (V./-)' + ^ / ( ^ ( ^ . * ) ) - ^-^{T - Tm)<^^ dQ.
(4,2.3)
Here, (J is a length scale associated with the 'microscopic interaction strength' (^ is the length such that two particles at a distance ^ apart will have the probability 1/2 of being in the same phase or state). a~^ is the measure of the depth of the double-well and is the indication of the barrier which must be crossed in the transition between the phases, a^/^^ is the 'correlation length', [S]E is the entropy difference between the phases at the equilibrium temperature per unit volume. All the terms in the integrand of (4.2.3) are in the units of energy per unit volume. Q C i?'^, n > 1 is a fixed region in space in which the phase-change process is taking place. In the phase transition problems with two distinct phases such as solid and liquid, / is a symmetric double-well function of 0 with two distinct minima, one for each phase. The last two terms in the integrand in (4.2.3)
88
Stefan Problem with Supercooling: Classical Formulation and Analysis
can be thought of as the 'local Helmholtz free energy density' and (V(/))^ is the 'basic interaction term' arising due to the interaction of atoms with a mean field created by other atoms [81]. The minimum of -F(0) is attained at that value of 0 which satisfies the following Euler-Lagrange equation derived under the condition that either 0 is prescribed or Vcp • n = 0 on the boundary of Q.
Liquid
1 0 0
Temperature
.//
/2 Solid ^ = -].0
Fig. 4.2.1. Enthalpy vs temperature. SF(^) ••e"^'
r
J
,
-1 0
Fig. 4.2.2. Continuity of the phase function.
.-i;/W + lf(r- Tm) = 0,
(4.2.4)
Here, the dash denotes differentiation with respect to the argument. 6F/S(j) is the 'variational derivative' of 0 which can be easily calculated (cf. [82]). Equation (4.2.4) holds for an equilibrium state and 0 is a function only of x, x G fl. Note that F is a function of (f) and not of T{x,t) and so T in (4.2.3) is fixed. For a transient or a non-equilibrium process, the free energy will not be minimum but will satisfy the following relation
^¥(^'*)^-¥^^(^'*»'
(4.2.5)
where (5 > 0 is the relaxation time [83]. Equation (4.2.5) is valid for the case when (p is not conserved, i.e., the volume integral of (t>{x,t) over Q is not prescribed. The basis of (4.2.5) is the standard idea of 'non-equilibrium thermodynamics' which assumes that cp returns to equilibrium with a 'force' proportional to the extent to which it was out of equilibrium. If (p is conserved, then
dt
SF {x,t) = div MgTcid{—-((t)(x,t))
(4.2.6)
where M is a coefficient related to diffusion or an interdiffusion coefficient which may depend on (t){x,t) and on the temperature [83, 84]. The second law of thermodynamics
89
4.2 A phase-field model using Landau-Ginzburg free energy functional
requires that along the solution paths, the free energy should necessarily decrease with time, failing which the model will be 'thermodynamically inconsistent'. If F in (4.2.3) is a function of (f) alone, then the second law of thermodynamics is satisfied but not when F is a function of the temperature and 0. The time derivative of F{(f){x,t)} gives FW.,t)}
= / g { ^ ( x , * ) } f rfO = - | / ( ^ ) ^ O < 0.
(4.2.7)
In order to derive (4.2.7) take F as in (4.2.3) and proceed with the first principles to obtain the time derivative of F. In calculating dF/dt, at some stage of calculations Green's formula is to be apphed to the volume integral of V(t)-Vd^ldt and if it is assumed that on the boundary of Vl the normal derivative of 0 is zero then we get (4.2.7). The second law of thermodynamics is satisfied even if 0 is conserved, provided M grad dF/S^ is zero on the boundary of Vt. Sometimes, since the order parameter 0 alone is not sufficient to describe the local state of the system, additional fields are introduced. One such extra field is temperature. If the heat conduction is very 'fast' then it can be assumed that the material is at a uniform temperature but this assumption is not valid if heat conduction is 'slow'. A second equation for heat transport in the material should be considered along with equation (4.2.4) or (4.2.6). An example of this type of modelling is the phase-field model which consists of the following system of equations. „ , . * , , 2 V V + i ( * - *') + '-^{T-T,).
(4.2.8)
In equations (4.2.8) and (4.2.9), (j) and T are coupled but in deriving (4.2.8) no couphng between 0 and T was considered. The double-well symmetric function / with distinct minima at 0 = ibl is taken as / = (0^ — 1)^. The parameter e and a are defined as £:^^ai/2, a = 8ie-
(4.2.10)
£ is a measure of the interface thickness and is of the order of 10~^ cm, a is the surface tension, S is the entropy per unit volume, k is the thermal diffusivity. The units of a, 5, a are as follows: (J : =
unit of Energy/(length)^"""^^ S :— unit of energy/{(length)'^-degree}
a:
unit of time/(length) , n = spacial dimension of Q.
=
(4.2.11)
At a deeper level of physics there are significant differences between the Stefan problem and the phase-field model in that the phase-field model incorporates the microscopic physics in an average sense and also takes into account macroscopic parameters. Stefan problem deals only with macroscopic modelling.
90
Stefan Problem with Supercooling: Classical Formulation and Analysis
Both the equations (4.2.8) and (4.2.9) hold good throughout Q and the sharp interface separating sohd and hquid phases has been replaced by a finite-width interface. Therefore there are no boundary conditions at the interface. The region corresponding to T = 0 is now given by — 1 < 0 < 1, and (f) = 0 is the center of the interface. Initial and boundary conditions for both 0 and T are to be prescribed. For example, on the boundary dQ of 0, we may have = 0,
dn
dT
(4.2.12)
= 0,
(4.2.13)
dn or any combination of these boundary conditions. Note that for both T < 0 and T > 0, (4.2.9) is well defined and the region T = 0 is not defined by the enthalpy but is identified by the phase function 0, — 1 < 0 < 1. The phase-field equations were first formulated in [85] on the basis of the model discussed in [80]. The last term in the integral in (4.2.3) can be taken in the form of a more general expression such as —2u^G{(f)) (cf. [86]), where u is the dimensionless temperature and u={T-Tm)/{l/Cp).
(4.2.14)
Here, 7 is a microscopic parameter related to a macroscopic parameter and has the dimension of energy per unit volume, G{(t)) is an increasing function related to the entropy distribution across the interface and 1
[S]E = 2^ j G\(l))d(t>.
(4.2.15)
-1
If G{(j)) = (/> and the units of energy and temperature are suitably chosen, then [S]E = 4 units of energy per unit volume/degree,
(4.2.16)
and -2u-fG{(f)) = -2u(j).
(4.2.17)
In Fig. 4.2.3. the last term on the r.h.s. in the equation to determine 0 has been taken as 2u. By defining e and a through (4.2.10) and a in units of free energy, the last term on the r.h.s. in (4.2.8) can be obtained. The capillary length o?o is defined as (cf. [87]). d, = ol{[S]EllCp},
(4.2.18)
4.2 A phase-field model using Landau-Ginzburg free energy functional
91
and it is easy to check that with the help of (4.2.18), (4.2.8) becomes ae'^
= s'V'+l{-') + ^^n.
(4.2.19)
Equation (4.2.19) has two length scales, viz., e and do. Note that there is only one length scale in the modified Gibbs-Thomson relation (4.1.1). Using (4.2.18), (4.1.1) can be written in terms of w as u = -doKc - adolVnl
(4.2.20)
In (4.1.1), specific heats, thermal conductivity and densities have been taken to be equal in both solid and liquid phases. This is generally done for convenience as the mathematics does not change significantly even if different thermophysical properties are taken in different phases. There are several advantages of phase-field models. A variety of phenomena ranging from motion by mean curvature to stable anisotropic crystal growth and single-needle dendrites are obtained by simply varying the parameters. Phase-field equations are amenable to numerical computations [88]. Computations involving selfinteractions of the interface do not pose difficulties in the phase-field model. The phasefield equations are guaranteed to have a smooth solution (0, T) if the initial and boundary data are sufficiently smooth (cf. [81, 89]). Rigorous mathematical analysis for obtaining Gibbs-Thomson relation by taking appropriate limits of some parameters was first presented in [81]. The 'Inner' and 'outer' expansions of the order parameter 0(a:, t) were obtained as ^ -^> 0 and by matching these expansions, it was proved that under suitable assumptions u{x) = -aoKji
-h 0 ( 0 , X e dQo.
(4.2.21)
where dQo is the center of the finite-width interface or where 0 = 0; Kc is the sum of the principal curvatures of the interface cp = 0 and CTQ is defined by the equation (To = ^ e
(4.2.22)
It was shown that (7 = aoH-0(^2).
(4.2.23)
Based on the mathematical procedure presented in [81], and by taking appropriate limits, modified Gibbs-Thomson relation was derived in [90] and the Gibbs-Thomson relation in [81]. In Figs. 4.2.3. and 4.2.4. (cf. [91, 90]), the appropriate hmits of the parameters to be taken, and the corresponding Stefan and Stefan-like problems obtained have been shown. Although the limiting cases obtained in Fig. 4.2.3. are covered in Fig. 4.2.4., the hmits are more explicitly defined in Fig. 4.2.3.
92
Stefan Problem with Supercooling: Classical Formulation and Analysis Phixfield equation
SurJaciimohinihMci
€chi.Akr,ii<M (Mfhix daman boindafia)
Scdin^imif u = 0 iritidl/CBtionbaindxj', /=0
Uohan bf mean cmciuft
U
7-»=--r |yi=Ji:[7ui;.»|
U
ScdingM u = 0 inMljcni<mbamikirj\
Fig. 4.2.3. Limits of phase-field equations I
4.2 A phase-field model using Landau-Ginzburg free energy functional
Ph6iefiiM\niha=0
""'"''i'-^Y-^V-rj
3urja(e tension M/O imtiu CpTt+-^j=KV'T Jv-n=KVT-ttl^
ivn=K7TnL = K7'T lv-n=KVTn|^ m£{r-g=-<JKc
Fig. 4.2.4. Limits of phase-field equations II
93
94
Stefan Problem with Supercooling: Classical Formulation and Analysis
The role of microscopic anisotropy in the macroscopic behaviour of the phase boundary was analysed in [92] using Landau-Ginzburg [80] free energy functional. The continuum limit for a lattice spin system with 'anisotropic interactions' J^ and spacing a^ leads to the following equation for the phase function 0.
^ f = |:4^g + ^(^-^V2«,
(4.2.24)
where d is the spacial dimension of the space, 6 is the relaxation time and u is defined by (4.2.14). The temperature and (p are coupled through an equation of the type (4.2.9) and ^i is related to Ji and a^. A modified Gibbs-Thomson law in polar coordinates (r, 6) was derived for the equilibrium temperature which has the form [S],u{r, 6) = -[a{0) + a"{e)]K - 5^0(6)1^9)
+ 0(^^).
(4.2.25)
Here, ^A(^) is a measure of the thickness of the interface and a{6) is the angle dependent surface tension. In [93] the basic phase-field model was extended to physical problems in which fluid properties such as velocity, pressure and density variations were incorporated along with heat properties and a unified and consistent derivation of equations was obtained. Since many phase-field models for solidification have been proposed in the literature, the phase-field model consisting of (4.2.8) and (4.2.9) will be called Standard phase-field model or SPF model.
4.3
Some Thermodynamically Consistent Phase-field and Phase Relaxation Models of Solidification
It was assumed in (4.2.3) that the energy functional F is a function of 0 alone, and that T is a constant. If a more general energy functional of the form F{{T{x, t), 4>{x, t)} = | { / ( r , ) + ^B\V4>{x)\'}dn, n
(4.3.1)
is taken, in which B is either a positive constant or a function of T but not of 0, then F will not always decrease on the solution paths and the thermodynamical consistency condition (4.2.7) will not be satisfied. However either the SPF model can be obtained as a particular case of some thermodynamically consistent models or the modified GibbsThomson relation can be derived independently of the SPF model. Several thermodynamically consistent models have been proposed in the literature (cf. [94, 95, 96, 97, 98]). Our interest here is only in briefly indicating the derivation of some thermodynamically consistent models from which the modifled Gibbs-Thomson relation can be obtained by a rigorous limiting process.
Some Thermodynamically Consistent Phase-field and Phase Relaxation Models
95
Entropy Functional The model proposed in [97] and described below is based on an entropy functional and is on the same hnes as the model proposed earlier in [94]. The starting point of the formulation in [97] is to define an entropy functional F{(f){x,t),e{x,t)} for a subregion Qg of a closed and bounded domain Q C R^ in which the phase-change is taking place.
F=l[s-\e'mr}dQ„
(4.3.2)
Here, S{e, 0) is the entropy per unit volume, (p is the phase function as defined in § 4.2, e{x,t) is the internal energy density and £ is a constant. e{x,t) satisfies the equation e H - V f = 0 in Qg,
(4.3.3)
where q is the heat flux vector. The time derivative of F can be calculated as follows:
'^-/(i)
+ e^V'
e+
^JUr.v
dS_ de
as dcp
/{( — j
q^e^Vct)
• £^V • {^V
+ e'V'
• n dAg.
(4.3.4)
dVL„
(4.3.5)
Here, Ag is the surface area of ^2^, dAg is the elementary surface area and n is the outward unit normal to Ag. The derivation of (4.3.5) from (4.3.4) requires a simple application of Gauss's theorem. The thermodynamical consistency of this entropy model requires that the local entropy production be non-negative. If the entropy flux through the surface of Q.g is subtracted from dF/dt^ then the entropy produced can be obtained. The consistency condition requires that dF
+/
q
T
-.
e^^V^-nj dAg > 0,
(4.3.6)
as the rate at which entropy is produced cannot be negative. q/T in the first term in the surface integral in (4.3.6) is the entropy flux due to heat flow (see (2.1.2)). T is the absolute temperature. The quantity e^cfNcj) is the entropy flux related to the changes in the phase function at the boundary of the subregion Vtg. Both solid and liquid phases have been taken at rest. If dF/dt from (4.3.5) is substituted in (4.3.6) and the relation 1
(4.3.7)
96
Stefan Problem with Supercooling: Classical Formulation and Analysis
is used, then we get the following consistency condition
Jhif)
+
©.-•^•'
ip}dng>
0.
(4.3.8)
The derivation of (4.3.7) deserves some discussion. In (2.1.15), the phase function 0 has not been considered. Therefore, as suggested in [94], it will be assumed that even when the free energy density G and the entropy density S depend on the order parameter 0 and the temperature T, they are concave in T and e, respectively (though not necessarily in 0) and they still obey 'Legendre transform relation' and its inversion [99]. G{T, 0) = inf [e(^, 0) - TS{e, 0)],
(4.3.9)
and 6(5", 0) T
S{e,(t)) = inf
G{T, T
(4.3.10)
The differentiations of (4.3.10) with respect to e and 0 give the following results dS_ {e. de
1 T{e,^
(4.3.11)
dG 1 (T(e,0),0) T{eA) dct>
(4.3.12)
and dS^
where T(e, 0) is the value of T at which the infimum in (4.3.10) occurs. For those values of T where no phase-change occurs, G is differentiable, and d{G{T^ct>)/T} dT
-e/T^
(4.3.13)
The consistency condition (4.3.8) gets satisfied if ^and 0 are chosen such that (4.3.14) and ^dS\
2v^o
(4.3.15)
Here, 8 is the relaxation time and is a positive constant, and MT could be a positive constant or a function of temperature but not of 0. In both [94] and [97], the starting point for the derivation of phase-held equations is the same entropy functional (4.3.2) but the consistency condition dF/dt > 0 is satisfied in different ways. We describe very
Some Thermodynamically Consistent Phase-field and Phase Relaxation Models
97
briefly the procedure adopted in [94]. The transient equations for (p{x,t) and e{x,t) have been obtained from (4.3.2) using the same arguments which lead to (4.2.5) and (4.2.6). While e{x^t) is conserved, (p may or not be conserved. The quantities SF/dcf) and 5F/6e, which are required in deriving transient equations for (p and e can be easily calculated, see [94]. Having obtained the transient equations, the consistency condition can be established as in (4.2.7). Note that the time derivative of the entropy functional contains both SF/S(f) and SF/Se and it should be nondecreasing. The quantity {dS/d(t))e in (4.3.15) is not convenient to handle. So it should be expressed in terms of {dG/d(t))T as in (4.3.12). Integration of (4.3.13) gives G{T,ct>) = Tl-J
^-^d(
+/{)],
(4.3.16)
where /(0) is an unknown function. A suitable definition of e(T, 0) agreeing with the classical definition of enthalpy can be developed if we take e{T,4>) =
esiT)+p{4>)L{T),
] (4.3.17)
= e^T) + [p(^) - 1]L(T), J where p(0) is an arbitrary function except that p(0) = 0 and p(l) = 1. In the present model, we take (j) = 0 is the solid phase and 0 = 1 is the liquid phase. L{T) = 6^(7') — es{T). When T = Tm^ L{Tm) = Im, where Im is the latent heat of fusion per unit volume. Any of the expressions for e given in (4.3.17) can be used in (4.3.16). If the second definition in (4.3.17) is used in (4.3.16), then we obtain
G(r, 4>) = TI-I
^d^
- \pi4>) - 1]Q(T) + m
j,
(4.3.18)
where
Q{T) = / r„
2 -«s«
(4.3.19)
By differentiating (4.3.18) w.r.t. 0, we obtain, dC\ — \
=-TQiT)p'{)+Tf'{4>).
(4.3.20)
Note that ei and e^ are functions of T and not of 0. It is now a simple matter to derive the phase-field model. Substituting e and (f from (4.3.17) and (4.3.14) in the energy equation (4.3.3), we obtain eUT)+p{mT)
+ {p(4>) - 1 ] ^ ( T ) = - V • [MrV ( i ) ] .
(4.3.21)
98
Stefan Problem with Supercooling: Classical Formulation and Analysis
If MT = KT^ and 0 = 0,1 in (4.3.21), then we get heat conduction equations in sohd and hquid phases, respectively. In order to obtain kinetic equation for 0, we substitute dS/d(l) from (4.3.12) in (4.3.15) and use (4.3.20). We obtain 6^ = Q{T)p\ct>) - f{ct>) + EV'CP .
(4.3.22)
Here, f{(p) is a double-well symmetric function with two distinct minima a,t (f) = 0 and 0 = 1 , and is taken as m
= 4>\l - 4>)\
(4.3.23)
It is not possible to determine p((/)) uniquely. The equations (4.3.21) and (4.3.22) can be presented in some more suitable dimensionalized forms (see [97]). The SPF model was obtained as a particular case of the phase-field model constructed in [95] subject to the conditions that the energy density vary linearly with the order parameter and the entropy be a non-concave function of the order parameter. Since SPF model can be obtained as a particular case of a thermodynamically consistent model, it can also be regarded as a consistent model. In [97], two phase-field models have been presented. One of them partially resembles the model in [85] and the other the model in [98]. Although SPF model cannot be obtained as a particular case of any of the models constructed in [97], it has been reported that by taking appropriate limits of the dimensionless equations derived in [97] and following the procedure discussed in [81], modified Gibbs-Thomson law can be derived. Some thermodynainically consistent phase relaxation models for supercooling The Stefan condition (1.4.11) imposed in the classical Stefan problem formulation does not arise in the phase-field models discussed above. Like an enthalpy formulation, phase-field model is also a continuum formulation, and (/> is defined as -1,
T<0,
(-1,1), T = 0,
(4.3.24)
1 , T > 0. The equations of the type (4.3.24) are valid throughout Q, which is the region under consideration. As mentioned earlier, in an enthalpy formulation, T = 0 does not give the middle of the mushy region but in the phase-field model, 0 = 0, gives the middle of the finite-size interface. In (4.3.24), the sharp interface separating solid and liquid regions is given by 0 = 0 and not by T = 0. It is well known that in the case of supercooling or superheating, a mushy region develops, and the free boundary progresses through the mushy region. Therefore to obtain an equiaxed or columnar growth of the interface or the amount of supercooling or superheating, the phase function 0, should be in some ways associated with the structure of the mushy region. It should be pointed out here
Some Thermodynamically Consistent Phase-field and Phase Relaxation Models
99
that T = Tm {or T = 0) is not the actual temperature of the mushy region. It is an average value of the temperature. The actual situation is shown in Fig. 4.3.1. Several mathematical models based on relaxation dynamics have been constructed in the literature in terms of 0 and T, with which modified Gibbs-Thomson relation, mushy region, dendritic growth, etc. (cf. [100, 101, 102, 103]) can be described. Most of these models are based on the weak formulation which is not the emphasis in this volume. We give below a classical formulation concerned with a family of thermodynamically consistent models of phase transitions. Phase diffusion is neglected in this formulation by assuming that no interfacial energy occurs [100]. This means that the term ^^(V(/))^/2 occurring in (4.2.3) is taken to be zero. Therefore, only a rate-type constitutive law governing the evolution of 0 accounts for the phase relaxation.
Enthalpy, H(T)
Fig. 4.3.1. ab-metastable state for the solid cd-metastable state for the liquid. The internal energy density e(T, (p) can be taken as e(T,0) = /o(T) + /i(0),
(4.3.25)
where equation (4.3.25) is the generahzation of the energy equation considered in the earlier phase transition models. If Cs = CL, then the specific enthalpy (or internal energy) is given by e = Ci/T + /0, O < 0 < 1.
(4.3.26)
As Cv = {de/dT)v > 0, we should have /^(T) > 0 in (4.3.25). The dash denotes diff"erentiation with respect to the argument. If 0 = 0 represents the solid phase, and 0 = 1 represents the liquid phase, which is a more energetic phase then we should have /i(0) > 0. For the sake of convenience, let e(T,,^)=T^F^(r)+T;^Fi(,^),
(4.3.27)
100
Stefan Problem with Supercooling: Classical Formulation and Analysis
where the functions FQ and Fi are assumed to be continuous with F((0) > 0 and T^ is the phase-change temperature which could be Tm or be different from T^. The energy balance equation has the standard form e(T, 0) H- div g^= r ; q = -k{T)VT,
(4.3.28)
where the dot denotes time derivative, r is the heat source density function and e is given by (4.3.27). To derive a constitutive equation for 0 it will be assumed that (j) is not an explicit function of the space variables which means that V0 = 0 or that no interfacial diffusion occurs which was present in (4.2.4). Therefore the constitutive relation for 0 can be assumed to be of the form 0 = /?(T,0),
(4.3.29)
where (5 is an unknown continuous function. The functions FQ, FI and [5 are unknown but not completely arbitrary. For example, if the system of equations (4.3.28) and (4.3.29) is thermodynamically consistent, then the Clausius-Duhem inequality should be satisfied, which implies that Q-VT G + ST-^ ^-—— < 0. 1^
(4.3.30)
Here, G is the free energy density which is related to entropy 5, e and T by the following relation. G(T, 4>) = inf{e(5,4>) - TS(e, 4>)}.
(4.3.31)
The time derivative of G gives 6^§T.f^l
(«,32)
Using (4.3.32) in (4.3.30), we obtain
The inequality (4.3.33) is satisfied if it is assumed that f)G
^
dC
+ 5 - 0, —/?(T, 0) < 0, k>^.
Substituting S = -dG/dT obtained.
(4.3.34)
in (4.3.31), and using (4.3.27), the following equation is
G-T—
= T'F;,{T)
+ T;^F,{>P].
(4.3.35)
It is easy to check that G{T, 4>) given by G(T,,^) = -TFo{T) + TB(^) - [T - TJF,(4>),
(4-3.36)
Some Thermodynamically Consistent Phase-field and Phase Relaxation Models satisfies (4.3.35). 5(0) is an unknown arbitrary function. (4.3.34) is satisfied if /3(T, 0) is taken as 1
101
The second condition in
dC
For the sake of simphcity g{T, 0) is taken as 5(T,,^) = F^WiV(T).
(4.3.38)
If P is continuous, and ^ > 0 and continuous, then N{T) should be continuous and N{T) > 0 for V T. Also F2 should be a monotonically increasing C^ function, i.e., ^2(0) > 0 for y (f). The constitutive equation for 0 can now be written as dC — {T,^) + N{T)F^{4>)4> = 0.
(4.3.39)
With constitutive equations (4.3.28) and (4.3.39), a thermodynamically consistent phase relaxation model is complete. By suitable choice of unknown functions FQ, F I , ^2(0) and N{T), several well known relaxation models can be derived as particular cases of this model (cf. [100]). Our interest here is only in analysing supercooling and superheating effects. As discussed earlier, when a phase is in a stable equilibrium, its free energy is minimum. Therefore, for stable solid and stable liquid phases, i.e., for 0 = 0 and (p = 1 at any fixed temperature, we have dG_
> 0 and —-
< 0,
(4.3.40)
as 0 = 0 and 0 = 1 are the end points of the interval [0,1]. If (j)e is any internal equilibrium point, i.e., 0e € (0,1), then dG
: 0.
(4.3.41)
If the mushy region is at equilibrium temperature T^, i.e., T^ = Tm then for all possible values of 0 G [0,1] there is equilibrium. From (4.3.36), at T = T^ = T^, we get G(T, 0) = -TFo(T) + TB{(t)).
(4.3.42)
From (4.3.41), dB{(l))/d(l) = 0 for 0 G [0,1]. Therefore 5(0) has a constant value in the interval [0,1]. In this case B = 7(0) = =
0, if 0 G [0,1] 00, otherwise.
(4.3.43)
102
Stefan Problem with Supercooling: Classical Formulation and Analysis
I{(p) is called a indicator function which has no classical derivative in R, but since / is a proper lower semi-continuous function, its subdifferential dl exists. The subdifferential of the function 7(0) is defined as a e dl{(t)) if and only if a((/) - 0 > ^(0) - HO for V ^ G /?, a e R.
(4.3.44)
With the help of the above definition it is easy to check that dl = H~^ where H~^ is the inverse of the Heaviside graph Hg defined below 0, x < 0
Hg[x)
= (0,1),2; = 0, =
(4.3.45)
l,a:>0.
It is easy to obtain H~^ or dl by using (4.3.44), and we have dl{x)
=
0, x ^ [ 0 , l ] ,
=
{0}, x G ( 0 , l ) ,
=
R+, x = 1, R- a x = 0.
(4.3.46)
To avoid cp taking values outside the interval [0,1], but to preserve strict minima at the ends 0 = 0 and 0 = 1, a double well function should be added to the indicator function. Therefore 5(0) should be of the form 5(0) = /(0) + Bi(0),
(4.3.47)
where Bi{(f)) is a smooth 'double-well function' forcing B to have only two strict minima at 0 = 0 and (p = 1. Depending on the physical situation, 7(0) or Bi{(f)) could be zero or non-zero. -Bi((/>) is generally taken in the form Bi{(t)) = iy(t)^{l-(t>)\ f / > 0 .
(4.3.48)
Fi((/)) in (4.3.36) has been assumed to be nondecreasing and it should have strict minima at 0 = 0 and 0 = 1, as G possesses these two minima. Fi{(f)) satisfies these conditions if it is taken as [100] Fi(0) = -0^(3-2(/)), A > 0 .
(4.3.49)
Superheating and supercooling effects When a sohd is superheated, 0 = 0 even at those points of the region where T > TmNote that superheating or supercooling is assessed with respect to Tm and not with respect to T^. Similarly, for a supercooled liquid, 0 = 1 even when T < T^. Therefore the free energy G must exhibit minima at 0 = 0 and 0 = 1 for all temperatures T such
Some Thermodynamically Consistent Phase-Eeld and Phase Relaxation Models
103
that \T — Tm\/Tm < e where e is a sufficiently small positive number. In particular, minima exists at T = T^ also. We first consider the case T > T^. For T > T^, the last term in (4.3.36) is nonpositive if Fi((/)) is taken as in (4.3.49). li B = Bi in (4.3.36), then G has an absolute minimum at 0 = 1. Fig. 2.1.1. suggests that the solid is superheated. For T < T^, the last term in (4.3.36) is non-negative and G (with B — B\) has an absolute minimum at 0 = 0. Fig. 2.2.1. suggests that the liquid is supercooled. Let u{T) = X{T — T^)/{^T), X > ly > 0. If Bi from (4.3.48) and Fi from (4.3.49) are substituted in (4.3.36) and G is differentiated, then we get BC
— = 4uT
(4.3.50)
If u{T) G (—1,1), then G takes two strict minima at 0 = 0 and 0 = 1 (check d^G/dcj)^ also) and superheating and supercooling can occur when T lies in the interval fiZk < T < ^ \ \X + u X-u)
.
(4.3.51) ^ ^
In (4.3.51), T^ has been replaced by Tm as supercooling and superheating are assessed with respect to Tm- It is clear from (4.3.51) that if the mushy region exists at T = T^^, then supercooling and superheating effects do not occur. In essence, the mathematical model described above consists of the following coupled differential equations. ^{T'F:,(T)
+ T^FM))
- div ( K ( r ) gradT) = r
(4.3.52)
and
j^iF^m+dm) + ^ f m ^ ^Jvi^^i w-
(4-3-53)
Equation (4.3.52) can be obtained from (4.3.27) and (4.3.28) while (4.3.53) can be obtained from (4.3.36) and (4.3.39). Due to the presence of the term dl{(/)), the l.h.s. in (4.3.53) is a multivalued function and the relation (4.3.53) cannot be written as an equation. The system consisting of (4.3.52) and (4.3.53) should be supplemented with the boundary and initial conditions. For example the boundary condition could be dT q'n = ~K{T)— = 0 on E = 5 0 x (0,t,),
(4.3.54)
where dCl is the boundary of the region Q under consideration and the initial conditions could be {T'F;,{T)
+ r^Fi(0)},.o = eo, (i^2(0))|.=o =
(4.3.55)
where CQ and 00 are known quantities. In Fourier's heat conduction law, k{T) = K{T) > 0, where K is the thermal conductivity. In irreversible thermodynamics.
104
Stefan Problem with Supercooling: Classical Formulation and Analysis
q = k{T)V (l/T). N{T) is a prescribed quantity. For example, in [95], N(T) has been taken to be T. If 7(0) = 0, then (4.3.53) is a classical equation. If K{T) = /T, a constant, and if suitable assumptions are made about various functions then the existence and uniqueness of the system (4.3.52) and (4.3.53) has been proved in [100].
4.4
Solidification of Supercooled Liquid W i t h o u t Curvature Effect and Kinetic Undercooling : Analysis of the Solution
In §§ 4.2 and 4.3 some phase-field models were considered. With their help supercooling/superheating effects can be discussed if appropriate limits of parameters are taken. These models provided rigorous justification of the incorporation of the modified GibbsThomson relation in the classical formulation when curvature eff"ect and kinetic undercooling are included. Many physical situations can be modelled by suitable choices of arbitrary functions occurring in phase-field models and phase-relaxation models. In this section we want to discuss issues like: (i) existence, uniqueness and well-posedness of solutions and (ii) blow-up and regularization of Stefan problems with supercooling. It may be made clear that by 'Stefan problem with supercooling' we mean classical formulations of either solidification of a supercooled liquid or one or two-phase solidification problems with modified Gibbs-Thomson relation.
4.4.1
One-dimensional one-phase solidification of supercooled liquid (SSP)
The mathematical model of this problem has already been discussed in (3.1.8)—(3.1.12). Because of its simplicity this problem has been extensively studied. It should be remembered that neither the curvature effect nor the kinetic condition has been incorporated in this model. It was mentioned in § 3.1.2 that ODP (equations (3.3.34)—(3.3.38)) can be transformed to SSP and vice-versa and c(j;, t) and T{x, t) are related through the equation (3.3.39). We shall now consider a more general ODP and take C(J:,0) =CO(X) , 0 < 2 ; < 1,
(4.4.1)
where CQ{X) could be different from 0.5(1 — x)^. From (3.3.39), we have
co(x) = fd^JiUv)
+ l)dv ; Co(x) = To{x) + 1,
(4.4.2)
4.4 SolidiEcation of Supercooled Liquid
105
and C:r{0,t) is given by (cf. [104]) t
c,{Q,t) = c',{0)-h jg{r)dr.
(4.4.3)
0
Here, g{t) is the same as in (3.1.10). The second equation in (4.4.2) suggests that Co{x) should be sufficiently regular. For the existence and uniqueness of solutions some compatibility conditions should also be satisfied by the initial temperature and the boundary conditions. If the boundary condition (3.1.10) is replaced by the boundary condition T(0,t) = F{t), then we obtain [104] t
c(0, t) = co(0) + I F{T)dT.
(4.4.4)
0
The essential diflFerence between the classical Stefan problems concerning solidification of a hquid and the solidification of a supercooled liquid is in the sign requirements of the temperature difference {T — Tm)- The temperature of a supercooled liquid is less than Tm^ whereas in the classical Stefan problem without supercooling, the temperature of the liquid is greater than or equal to Tm- As mentioned earlier, whenever the classical solution of a Stefan problem is discussed here, it is implied that the solution satisfies some conditions of the form (i) - (iv) given in § 3.1.1. Depending on the problem under consideration appropriate changes have to be made in these conditions. A global-in-time classical solution is known to exist for the one-dimensional Stefan problem under suitable data assumptions (cf. [57, 75]). If the liquid is supercooled or if the solid is superheated, a solution may still exist for a short-time but a finite-time blow-up (explained below) may occur, which prevents continuation of the solution. Assuming that a solution of a Stefan problem with superheating/supercooling exists, some important questions to be investigated further are: (A) Can a classical solution of the problem exist for an arbitrarily long time ? (B) Does there exist a constant ^6 > 0 such that hmt^i^_ S{t) = 0? Here S{t) is progressing towards x = 0. (C) Does there exist a constant tc > 0 such that S{t) > 0 for i G (0, tc] and lim,_,^ S{t) = -oo? Question (B) pertains to finite-time extinction and (C) pertains to the blow-up of a solution defined below. Fimte-time blow-up: If S{t) is the free boundary in a free boundary problem, and if at some time t = t* < oo, S{t^) > 0 and liminft._^f*_ S{t) = —oo, then it is said that a finite-time blow-up has occurred. Essential blow-up: If the solution of the free boundary problem cannot be continued beyond the blow-up time, then the blow-up is called essential. It may be pointed out
106
Stefan Problem with Supercooling: Classical Formulation and Analysis
that blow-up is not a physically reahstic phenomenon and it indicates defects in the mathematical model of the problem. Non-essential blow-up: It may be possible to continue the solution of the free boundary problem beyond the blow-up time t = t^ with S{t^) and hm^-^t*- T{x,t) = TQ{X) as the initial data. In this case we have a non-essential blow-up. Many studies are available on the analysis of SSP. Each study contains several results. Only a few of these can be presented here. In [59], [104] and [105] the boundary condition (3.1.10) in SSP, has been replaced with the following: T,(0,i) = 0, 0 < ^ < ^ , .
(4.4.5)
Let Q be a constant defined as 1
Q = J{TQ{X) + l)dx.
(4.4.6)
0
The quantity {l-\-To{x)) in which the latent heat has been suitably scaled and taken as unity, represents the sum of the latent heat and the sensible heat. Q can be interpreted as the global energy of the supercooled liquid at ^ = 0 and should be greater than or equal to zero. Q plays an important role in analysing questions (A), (B) and (C) mentioned above. Let TQ{X) be non-positive and continuous continuous dX X = 1, then the supercooled sufficiently small time t
in [0,1]. If To(l) = 0 and TQ{X) is Holderproblem possesses one solution valid for and well-posedness of the solution can also —1, then SSP has no solution [75].
Some of the results reported in [75], [104] and [105] are summarized below in the form of propositions 4.4.1.—4.4.5. It should be remembered that in the discussion pertaining to these propositions, the term SSP means the mathematical formulation (3.1.8)—(3.1.12) in which (3.1.10) has been replaced by (4.4.5). Similarly ODP would mean the problem obtained by transforming the new SSP with the help of the transformation (3.3.39). Proposition 4.4.1. If the closure of the set {To{x) < —1} is either void, or if it is compact and contains the origin, then in the solution of SSP the conditions Q > 0, Q = 0 and Q < 0 are necessary and sufficient for global existence, finite-time extinction and blow-up, respectively. Blow-up always occurs when Q < 0 while Q = 0 is necessary for a finite-time extinction. If the constraint c > 0 is not added in ODP with initial concentration given by (4.4.1), then the free boundary in both SSP and UODP (unconstrained ODP) coincides and both will have a finite-time blow-up at a time at which the negativity set of c(x, t) reaches x = S{t). Fort e (0,t*), define N{t) = {x:0<x
< S{t), c{x, t) < 0}.
(4.4.7)
4.4 Solidification of Supercooled Liquid
107
In [104, 105], several results concerning blow-up in SSP and UODP have been reported. Some of them are given below. Proposition 4.4.2. If N{t) 7^ 0, then (i) for any ^2 ^ (^i,^*), ^(^1) CC N{t2), i.e., the negativity set expands, (ii) if for some i > ti, the boundary dN{i) singular as t ^^ i,
touches the free boundary then S{t) is
(iii) the case above actually occurs. Proposition 4.4.3. A necessary and sufficient condition for an essential blow-up at the point {S{i),£) is that the point belongs to dN{i). Proposition 4.4.4. A necessary and sufficient condition for having S{t) singular at a point {S{i),i) is that the set {{x,t) : T{x,t) = —1} reaches the free boundary at such a point. Proposition 4.4.5. There are initial data such that ODP corresponding to SSP with (4.4.5) admits global solutions with limt_to *5'(^) = —00 for some ^0 > 0. The last proposition is a case of a non-essential blow-up and in [104] this case has been demonstrated with the help of an example.
4.4.2
Regularization of a blow-up in SSP by looking at C O D P
The question which will be discussed now is, 'how to stop blow-up in SSP'? This is called regularization. Note that there are methods other than the one discussed below for regularization. Let us first examine which physical quantity is responsible for the blow-up. The integral S{t)
E{x, t)= J (T(e, t) -h l)d^ = - c „
(4.4.8)
X
represents the thermal energy in the layer {x,S{t)) (cf. (3.3.39)). E{0,t) = E{0,0) = Q (cf. (4.4.6)). Here, c{x,t) is the concentration in ODP. If the mean energy E in the supercooled layer is defined as S{t)
S{t)
E{x, t) = {S(t) - x)-' I di I {T{r], t) H- l)dr], = {S{t)-x)-'c(x,t),
(4.4.9) (4.4.10)
then blow-up occurs if and only if E{x,t) is negative at some point {x,t). This implies that at the blow-up point c(x, t) < 0. Thus any regularization of SSP or UODP has
108
Stefan Problem with Supercooling: Classical Formulation and Analysis
to prevent E{x,t) from taking negative values by shifting UODP to CODP. It seems reasonable to conjecture that this argument holds for multi-dimensional problems also (cf. [105]). The absolute minimum of E{x, t) is E{0, t) or Q. Blow-up occurs if and only if Q < 0. If (5 < 0, then it implies that the system does not have enough energy in the form of latent heat to cancel out the 'cold' of the supercooled liquid. Another way to look at the blow-up points is to look at the roots of the equation T(x, t) -hi = 0. Either the set of points {x, t) for which T{x, ^) -1- 1 = 0 is empty, or it has a positive distance from the free boundary. In the latter case, if this set reaches the free boundary at t = to then \im.t-^to S{t) = — oo (cf. [75]). For t < to, S{t) is bounded. This imphes that there is too much 'cold' too near the free boundary, and blow-up occurs because the energy cannot reach the point of blow-up by diffusion. To stop finite-time blow-up in SSP some mathematical techniques have been suggested in [104] which consist of modifying UODP to CODP provided CODP is formulated classically, with a new free boundary appearing each time c{x, t) becomes zero. We can call it 'nucleation' of new free boundaries. In this case we have always a finite-time extinction. This 'nucleation' of new free boundaries can be achieved in SSP also. Let 0 = 0, Co > 0. This implies that Ca;(0, t) = 0. There is no change in the energy of the system in this 'nucleation' but the creation of the new phase boundaries rearranges the energy fluxes in such a way that the blow-up that would occur due to the deficit of energy near x = S{t) is prevented by a counter-balancing surplus near x = 0. There are techniques such as incorporation of curvature effects and kinetic undercooling for the regularization of blow-up during solidification of a supercooled liquid. Curvature effects cannot be included in SSP as it is a one-dimensional slab problem. If the kinetic condition is also excluded in the formulation, then we have to resort to other techniques for regularization such as nucleation described above. Some of the results obtained in [105] have been extended to a two-phase problem considered in [106]. Let us assume that there is some sohd region (finite or infinite) to the right of x = S{t) in SSP whose temperature is Ti{x, t) < 0 for t > 0. The boundary and initial conditions are such that T{x,t) < 0, t > 0 is guaranteed till the solution exists. The following proposition, which is false for the one-phase SSP problem, is true for the two-phase problem. Proposition 4.4.6. If Q = 0, an essential blow-up will occur, while, for finite-time extinction Q > 0 is necessary. The above result has been proved by obtaining an integral equation for S{t).
4.4 Soliditication of Supercooled Liquid
4.4.3
109
Analysis of problems with some changes in t h e initial and boundary conditions in SSP
Let us consider the formulation (3.1.8)—(3.1.12) with the boundary condition T(0,t) = / ( t ) , 0
(4.4.11)
By using the transformation (3.3.39) and using (4.4.4), a formulation in terms of c{x,t) can be obtained. Let us assume that in the transformed problem c(x,0) = co(x), 0 < x < 1.
(4.4.12)
The following results have been proved in [107]. Proposition 4.4.7. Assume that for some XQ > 0 the following condition is satisfied. co(x)<0, for X G ( l - x o , l ) .
(4.4.13)
Then the solution of SSP with (3.1.10) replaced by (4.4.11) does not exist. Proposition 4.4.8. Assume that there exists XQ > 0 such that the initial temperature To{x) > - 1 , To(x),^ - 1 in (1 - XQ, 1). Then SSP with (4.4.11) has at most one solution in the local sense. Proposition 4.4.9. Assume that there exists XQ > 0 such that To{x) > - 1 for x in (1 — xo, 1). Then SSP with (4.4.11) possesses a unique local-in-time classical solution. Existence can also be proved in a special case in which lim^;^!- inf To(x) > — 1. The convexity of the free boundary in SSP has been proved in [108] with no flux boundary condition imposed at x = 0, i.e., in (3.1.10), g{t) = 0. In [109] also, the convexity of the free boundary has been established but under special assumptions on To(x). The initial data considered in [108] is of the form To(x) < 0, X G [0,1]; To(l) = 0; To(x) G C'[0,1].
(4.4.14)
The main tool used in [109] in proving the results was the introduction of the function ^(x,^) = T^JT^,
(4.4.15)
and the study of its level curves and this was done in [108] also. SSP can be formulated in terms of i;(x, t) which will not be discussed here. Let M = {x : X G [0,1], To'(x) 7^ 0.}.
(4.4.16)
If To(x) ^ 0, then M is not void. Set P(x) = To(x)/To'(x), X G M. Out of the several results reported in [108], we present only one here.
(4.4.17)
no
Stefan Problem with Supercooling: Classical Formulation and Analysis
Proposition 4.4.10. Assume that (4.4.14) is satisfied, and P{x) is non-negative and non-increasing in M. If P ( l ) > To(l), then S{t) > 0 for any t G (0, t^) and SSP with ^(^) = 0 in (3.1.10) is solvable for any arbitrary t^ > 0. Some results can be obtained for ODP also by following the approach adopted for SSP. In particular it can be proved that the free boundary in ODP with CQ{X) = 0.5(1 — x)^ is concave. See [108] for further details. In [110], SSP with (3.1.10) replaced by the prescribed temperature condition T(0, t) = f{t) has been considered. The questions (A), (B) and (C) raised earher in this section have been analysed. Several results have been obtained in terms of a quantity R{t) defined as 1 ^ * R{t) ^ _ + fxTo{x)dx + ff{T)dT. 0
(4.4.18)
0
The motivation for considering R{t) comes from the equation
(s(t))'-i ^-^-^
}
\
= / xT^{x)dx + / i(T)dT 0
0
T / xT{x, t)dx.
(4.4.19)
0
When S{t) = 0, we have R{t) — 0. Equation (4.4.19) and several other equations of this kind can be derived by using the following Green's identity (cf. [57]). y / ( i ; P ( T ) - TP%v))dxdT = J [{T^v - Tv,)dT + k-^Tvdx] , Dt
(4.4.20)
dDt
where P is the heat operator k~^Txx — Tt and P* is its adjoint operator, Dt = {{x^t) : 0 < X < S{t),0 < t < t^}. By choosing v and v^ suitably, several relations of the type (4.4.19) can be developed. Proposition 4.4.11. Assume that the solution of SSP (with temperature prescribed in the place of (3.1.10)) exists, and To{x) and f{t) satisfy the following hypotheses: (i) To{x) < 0 is an increasing function in [0,1], (ii) f{t) < 0 is a decreasing function of ^, ^ > 0. If case (C) (§ 4.4.1) occurs, then it implies R{tc) < 0 {tc is the same as defined in (C)). The following convective boundary condition at a: = 0 in SSP was considered in [111] in the place of (3.1.10). K^(0, t) = h(T(0, t) - g(t)), git) < 0, 0 < ^ < ^*. (4.4.21) ox The questions (A), (B) and (C) (§ 4.4.1) were investigated and in answering these questions, the key role is played by the quantity Qi{t) defined as Qi(0 - 1 + ^ + / ( I + l3x)To{x)dx + Jf3G{T)dT,
(4.4.22)
4.4 Solidification of Supercooled Liquid
111
where P = h/Kb is a dimensionless parameter, b has the dimension of length with which the space coordinate can be dimensionahzed to obtain the supercooled region as 0 < a: < 1 and
We present here one of the results reported in [111]. Proposition 4.4.12. If the solution of this changed SSP with (4.4.21) exists and the initial and boundary data satisfy the following conditions (i) To{x) > M{x - 1), and T(5(x) > 0 , 0 < x < l ,
0<M<1,"
(ii) G{t) > - M , t > 0,
\
(4.4.24)
( i i i ) n ( 0 ) = /?(T(0)-p(0)), and there exists a time tb such that Qi{tt) = 0, then t = tb is the time in the case (B). The solidification of a supercooled liquid has been studied in geometries other than a slab. In a one-dimensional radially symmetric spherical geometry or in a one dimensional radially symmetric cyUndrical geometry, in principle, curvature effects can be included. The curvature effect and the kinetic condition have in general a regularizing effect. If curvature effect is not included in cylindrical and spherical geometries, then the questions (A), (B) and (C) become more meaningful and should be investigated. This has been done in the next two problems. These problems can be easily formulated on the same lines as the formulation given in (3.1.8)—(3.1.12). One-dimensional radially symmetric solidification of a supercooled liquid in the spherical region ro < r < 1, ro > 0 was considered in [112]. The quantity Q2{t) given below plays a key-role in the study of questions (A), (B) and (C). 1
T
Q2{t) = l + 2> j r^To{r)dr - Zrl j g{T)dT. ro
(4.4.25)
0
Here, To(r) is the initial temperature and g{t) is the prescribed flux at r = ro in the spherical geometry. A result concerning finite-time extinction is as follows. Proposition 4.4.13. If To(r) < 0, ro < r < 1, and continuous and To(r) > —hi(\ — r), /ii < 1, 0 < g{t) < hi for ^ > 0, then case (B) occurs and Q2{tb) = ^oSolidification of supercooled liquid in a one-dimensional radially symmetric cylindrical region ro < r < 6 was considered in [113] with temperature prescribed at r = ro- A flux prescribed case can also be discussed in a similar way. One existence and uniqueness result is given below. Proposition 4.4.14.
Let the initial temperature To(r), ro < r < 6 be such that To(r)>-l,
b-do
(4.4.26)
112
Stefan Problem with Supercooling: Classical Formulation and Analysis
for some do G (0, b — vo]. Then the solution of the cyhndrical problem exists in the interval 0 < t < t* for some t* > 0. The case To{b) = - 1 is allowed as long as conditions in (4.4.26) are fulfilled. Uniqueness of the solution can be proved as in [107]. The SSP considered in [114] is ODP transformed into SSP. The initial concentration in ODP is 0.5(1 — x)^ but at x = 0 non-zero flux is prescribed which is a monotonically increasing function of time with Cx{0,0) = —1. This ODP can be transformed to SSP with the initial temperature zero and the flux at x = 0 given by Tr(0, t) = g{t), 0 < t < t^. It is assumed that g{t) is a non-negative piecewise continuous function in (0, oo), bounded in every interval (0,t), t > 0. The questions (A), (B) and (C) have been analysed for the transformed SSP. By using an appropriate Green's identity, the relation given below can be obtained. t
S{t) = l -
[ g{T)dT0
s{t)
f T{x,t)dx,
0 < t < t,.
(4.4.27)
0
A unique solution of the transformed SSP exists if and only if t
[ g{T)dT < 1 for any t > 0.
(4.4.28)
0
For other results see [114].
4.5
4.5.1
Analysis of Supercooled Stefan Problems with t h e Modified Gibbs-Thomson Relation Introduction
A supercooled liquid is in a metastable state, and, if left to itself, will remain in that state for a long time. In the absence of curvature effect or kinetic undercooling in the equilibrium temperature, supercooled liquid has a destabilizing effect on the planar phase-change boundary. Imagine a small solid protrusion into the supercooled liquid at the planar interface. Suppose that the temperature of the supercooled liquid is decreasing as we move away from the planar front. Let the equilibrium temperature be Tm which is the ideal phase-change temperature. This solid protrusion will have a large temperature gradient. According to Stefan condition (1.4.22), its growth velocity wih be greater than that of its neighbours on the planar front, and so this protrusion wiU grow and not dye. This indicates instability. The effect of surface tension is to stabilize the phase-change boundary. Since the surface tension decreases the equilibrium temperature, any protrusion at the planar free boundary gets liquified. Without surface tension, supercooling could lead to a totally unstable system. In real physical systems when supercooling is present, and which is generally the case, local instabilities such as dendrites will be present. These are counter
Analysis of Supercooled Stefan Problems with Modified Gibbs-'lhomson Relation
113
balanced by the nonlinear stabilizing effects of surface tension. In some sense, these arguments also hold for kinetic undercooling as well [86] that kinetic undercooling has a stabilizing effect in the sense that in the perturbation of the planar boundary an unstable mode remains unstable but with smaller amplitude. Large surface tension is incompatible with large curvature as large surface tension in supercooling implies that the deviation of the new equilibrium temperature from T^ is negative and large. This means that protrusions will be dissolved. This also means that the interface is nearly planar and the curvature is small. Therefore if the initial and boundary conditions are suitably chosen, then supercooling can be restricted. Surface tension is an equilibrium phenomenon, as it arises in equilibrium thermodynamics, but kinetic undercooling is a non-equilibrium phenomenon. It may be noted that in the Stefan problem, phase-change boundary is moving but it is assumed that there is an instantaneous equilibrium so that laws of equilibrium thermodynamics can be applied.
4.5.2
One-dimensional one-phase supercooled Stefan problems with the modified Gibbs-Thomson relation
Consider the formulation given in (3.1.8)—(3.1.12) with g{t) = 0 in (3.1.10) and (3.1.11) taken as T{S{t),t)
= £S{t), £ > 0, S{t) < 0.
(4.5.1)
Equations (3.1.8), (3.1.9), (3.1.10), (3.1.12) and (4.5.1) constitute our new SSP. In equation (4.5.1) kinetic undercooling has been incorporated. Note that since S{t) is moving towards x = 0, S{t) is negative. In the problem considered is [115], 5(0) = 0 and S{t) > 0 which is different from our formulation. This change in essence does not affect the results of [115]. It hgis been established in [115] that if the initial temperature To{x) is such that TQ{X) is continuous in 0 < a: < 1 and TQ{0) = 0 then the kinetic undercooling will have a regularizing effect and blow-up will be prevented. In physical terms, the kinetic undercooling term allows more energy to enter the liquid at the moving boundary than would be the case if this energy is supphed by the latent heat alone (as in SSP without kinetic undercoohng). The rate of change of energy is given by (cf. equation (4.4.8) for E{x,t)) S{t)
dE dt
-d j - {T{x, t) + l}dx = sS''.
(4.5.2)
As £ > 0, there is an additional energy eS'^ available which compensates any energy deficit in TQ[X). The kinetic undercooling term in (4.5.1) imposes a limit on the speed of the interface. For some results concerning a one-dimensional two-phase problem and a semi-infinite problem, see [115]. A one-dimensional supercooled problem in a semi-infinite region 0 < x < oo has been considered in [116]. The region under consideration is S[t) < x < oo, t > 0 and
114
Stefan Problem with Supercooling: Classical Formulation and Analysis
S{0) = 0. Let To{x) < 0, 0 < x < co, To{x) e C^[0, oo) be the initial temperature of the supercooled liquid and To(x) ^ - 1 - ^ as X -^ 00,
(4.5.3)
where ^ is a constant. If kinetic undercooling is considered, then T{S{t),t) = -£S{t),
£ > 0, S{t) > 0, t > 0.
(4.5.4)
No fixed boundary condition at x = 0 is required as the region x < 0 is ice at time t = 0. Several cases for ^ > 0, = 0, < 0 have been analysed when e > 0 and s = 0. When s = 0, and ^ > 0, a finite-time blow-up occurs. When e > 0 and ^ < 0, similarity solutions exist and S{t) = ^yft (for (5 see [116]). When s > 0, and ^ > 0, travelling wave solutions exist with S[t) = St/e. If £ > 0, and 6 = 0; S{t) ~ df^, | < a < 1, where d is a constant depending on several parameters including s. A radially symmetric spherical problem in the region S{0) < r < oo, S{0) y^ 0 was also considered in [116]. Both surface tension and kinetic condition have been considered so that T{S{t),t) - -eS - 2a/S,
5 > 0, t>0.
(4.5.5)
Note that in this case the curvature Kc = l/r and the free boundary is r = S{t). Initial temperature is of the same type as in (4.5.3). li e = 0 and ^ > 0, then even with surface tension a finite-time blow-up occurs. However if s > 0, and ^ > 0 , — O o r < 0 , finite-time blow-up does not occur. For other results see [116]. In [117], the effect of both kinetic undercoohng and latent heat on the solution has been analysed. Consider SSP described by (3.1.8)—(3.1.12) in which instead of (3.1.10), the boundary condition T(0, t) = f{t) is prescribed. The interface conditions are T{S{t),t) = £S, s>0,
5 < 0, and T,{S(t),t)
= -IS,
(4.5.6)
To(x) and f{t) are continuous non-positive functions such that To(0) = /(O). When 5 = 0, and / < 0, (/ is the latent heat) we have a one-phase Stefan problem concerning solidification. When e = 0 and / > 0 we have a supercooled Stefan problem. In the latter case, if £ > 0, then it will be a supercooled problem with a kinetic condition. Several cases have been considered for different signs of e and / but our interest at present is in the case ^ > 0 and / > 0 for which the following result has been proved in [117]. Proposition 4.5.1. If To(x) and f{t) satisfy the assumptions stated above, then the supercooled problem with (4.5.6) in which e > 0, and / > 0 admits a unique solution and the free boundary S(t) is a smooth function in (0, t^) where t^ is such that S{t^) = 0 and S(t) > 0 for t
Analysis of Supercooled Stefan Problems with Modified Gibbs-Thomson Relation
4.5.3
115
One-dimensional two-phase Stefan problems with the modified Gibbs-Thomson relation
Although a two-phase Stefan problem with the modified Gibbs-Thomson relation can be easily formulated, for the sake of better understanding of the results, we present here the formulation of a two-phase Stefan problem with a kinetic condition.
~dt' ^ ^'~dJ''
'"^ ^' ^ ^^""'^^ : 0 < :r < S{t), 0< t < t,},
(4.5.7)
^
'"^ ^ ' " ^^''' ^^ • ^^^^ < ^ < 1, 0 < ^ < ^*},
(4.5.8)
" ^ ^ ^ '
Ts{S{t)-
0,t) = TL{S{t)+ 0,t) = T{S{t),t),
0
S{t)=g{T{S{t),t), ^(0) = So, 0<So<
{Ks^
I
^^
(4.5.10) 1,
= lS(t), / > 0, 0 < S(t) < constant,
- KL^\
(4.5.9)
(4.5.11)
(4.5.12)
^^ L=sit) T{x, 0) = To(a;), 0 < x < 1,
(4.5.13)
TL(1, t) = hL{t), Ts{0, t) = hs{t).
(4.5.14)
This solidification problem has been considered in [118]. The main interest in [118] is to establish the existence and uniqueness of the classical solution of the problem if g{T) is of the form (4.5.15) g{T) € C\R) and g{T) = T'^,me N. The condition (4.5.12) is satisfied at x == ^o also. hi{t) G C"^[0, t*], i = S,L,\ < ai < 1, To{x) e C^'[0,So]nC^'[SoM 2 < as < 3, To(0) - hs{0), To(l) = /IL(0) and So satisfies a constraint (cf. [118]) which involves the L°°-norms of the initial and boundary temperatures. No sign constraints have been prescribed on the initial and boundary data. It has been proved that there exists a t^ such that liuit^t^ S{t) = 1. Travelling wave solutions of the problem have also been obtained. A similar problem with g{T) e C^{R) and \g{T)\ < a(|T|"^ + 1), where a and m > 1 are positive constants, was considered in [119] and local existence of the classical solution was proved under some assumptions some of which are the same as in [118].
116
Stefan Problem with Supercooling: Classical Formulation and Analysis
A two-phase problem in —oo < a: < oo with kinetic undercoohng condition (4.5.10) has been considered in [120]. It was proved in [120] that if \g{T)\ < Me^'^' for some M > 0 and 7 E (0,1/4), then a global solution to the problem (4.5.7)—(4.5.14) exists under suitable assumptions. If g{T) > ae^^'-^' for some constant a and large 71, then blow-up in finite time may occur. A critical value of the exponent 7 was obtained in [121] as 7 = \/27re such that for 7 G (0, V27re), a global-in-time solution exists and blow-up in finite time occurs if g{T) > ae^'-^L In [122], the problem (4.5.7)—(4.5.14) was considered and a single function ip{x^t) represents both initial and boundary conditions. T(x,0-V^(x,^), {x,t)ed'Qt^,
(4.5.16)
where d'Qu = {{x,0):xe
(0,1)} U {(0, t):te
(0, t,)} U {(1, t):te
(0, t,)}.
It was proved that if g{T) G C^(i?), 0 < /? < 1, ip e W^^\Qt^), g > 3 and the following conditions are satisfied (any one out of (4.5.17) and (4.5.18)), g{T) < 0, T < mi, g{T) > 0, T > ms for some mi, ms G R, 1^(7")I < CLi\T\ + a2, ai and a2 are constants, gW.^o)
= 9W.=i) = 0,
(4.5.17) (4.5.18) (4.5.19)
then a global classical solution exists such that S{t) G C^+^/2[0, t*],_A G (0,1). VK^^'HOtJ, 0' > 1 is the Sobolev space of functions u{x,t) in the domain Qt^{Qi UQ2 x (0,t*)) with bounded norm Mw^^\Qu)
= ( / d^l' + l^-l' + l ^ - l ' + lutndxdty/^.
(4.5.20)
Replace the condition (4.5.10) in problem (4.5.7)—(4.5.14) by the following kinetic condition T'{S'{t),t)
= -eS{t),
e>0,0
(4.5.21)
where T^ and S^ are written in place of T and S to signify that both T and S now depend on 6. This problem has been considered in [123], and the results given in Proposition 4.5.2. have been estabhshed. Proposition 4.5.2. tions.
Let To(x), hs and hi satisfy the following smoothness assump-
To(x) G C^O, So] n C'[So^ 1] n C[0, l] and hiihs) G C'{R) 0 L^oiR).
(4.5.22)
Analysis
of Supercooled
Stefan Problems
with Modified
Gibbs-Thomson
Relation
117
The consistency conditions at x = 0, and x = 1 for the initial temperature and boundary conditions are satisfied and the functions hs and hi do not change sign for ^ > 0. Under the above assumptions a unique classical solution (T^, S^) (in the sense of § 1.4.6)of the problem (4.5.7)—(4.5.14) with (4.5.21) taken in the place of of (4.5.10) exists for every £ > 0 and for some tl > 0. Either tl = +(X) or min{l - S'{tl), S'{tl)} = 0. Depending on the boundary conditions, S could be greater or less t h a n zero. By considering a sequence of problems {T^"", 5"^"}, ^n > 0, ^ = 1, 2 . . . and taking the limit £n -^ 0, the existence of a weak solution has been established in [123] and it has been shown that T{S{t),t) -^ 0 for a.a. t G (0,t*). Suppose the initial d a t a is such that for some Si G [6'o, 1] To{x){x -Si)>0,
xe
[0,1], X + Si, So.
(4.5.23)
If (4.5.23) is satisfied, then a supercooled liquid phase exists between stable solid and stable liquid phases. It has been proved in [123] t h a t the simply connected supercooled liquid region disappears in finite time, after which the solution becomes a classical one. In (4.5.23), the degree of supercooling is not prescribed. It was pointed out earlier for a onephase supercooled problem (cf. Proposition 4.4.1.) t h a t the negativity set {To{x) < — 1} is responsible for a blow-up (roughly speaking). T h e initial d a t a given below gives rise to a discontinuity in the free boundary. Let 5j^ and S'f G (ASQ, 1) be such that To{x) < - / , xe
To(x) > - / , xe
(ST,*S+),
(4.5.24)
(5-0, 5 r ) U (5+, 1),
(4.5.25)
where / is the latent heat, and / > 0. Under suitable assumptions, it has been shown in [123] t h a t the free boundary is smooth everywhere except at a point t = t < t^. At this point S{t) may have a j u m p . S{t -(- 0) — S{t — 0) > 0 will be exactly equal to the length of the interval in which T{x, i — 0) < —I. In the Neumann solution (1.3.11)—(1.3.16), the velocity of the interface (cf. (1.3.13)) is infinite at ^ = 0 which seems to be inconsistent with the assumption of local thermodynamic equihbrium at a constant freezing temperature. If the kinetic condition (4.5.26) is incorporated in (1.3.5), ps — pi and in the place of / = {/ + {Ci — Cs)Tm}, I is taken, then we get a problem considered in [123]. Let Ts{S{t),t)
= TL(S{t),t)
- -£S{t),
£>0,
S>0.
(4.5.26)
Under suitable assumptions, the existence and uniqueness of the Neumann solution with the condition (4.5.26) and other changes have been proved. It has been shown that T(x, t) G C^iQi) n C°^(02) and S(t) G C°^(0, t,), where Qi{Q2) = {x>0,xS{t) > 0(< 0), 0 < t < ^=,}. S{t) > jt for t G (0,^*), where 7 is a positive constant. An analytic solution has been obtained which is validated by a numerical solution obtained
118
Stefan Problem with Supercooling: Classical Formulation and Analysis
by a finite-difference scheme [123]. An equation for the phase-change temperature has been obtained in [124] by considering the entropy production in the interface, pressureinduced undercoohng, and viscosity-induced variations in the freezing temperature. A fairly general expression for the latent heat depending on the thermal expansion coefficient, temperature, pressure, etc., has also been obtained. In deriving phase-change temperature and latent heat expressions, non-equilibrium thermodynamical considerations have been used. Dissolution of a spherical ice ball in supercooled water has been discussed in [125]. This one-dimensional radially-symmetric spherical problem is closely related to a Stefan problem with modified Gibbs-Thomson relation. In [125], a weak solution has also been discussed. We give below only the classical formulation of the problem of dissolution of a solid ice ball of radius S{t) in a supercooled liquid. Consider the following equations.
r ( r , 0 ) = To(r) < 0, 0 < r < ro, %{0) = 0; Ti(ro,<) = Ti{t), t > 0,
(4.5.28)
TsiS(t), t) = n{S(t),
(4.5.29)
i) = - J - ^ + / ? ^ , r = S(t), t > 0; 5(0) = 1,
^ - f ^
= - / f
onr = S(t),t>0,
(4.5.30)
Here, /3 > 0 is the relaxation parameter arising in the kinetic condition, the surface tension in (4.5.29) is taken unity and Ql = {(r,t) : 0 < r < S{t); 0
S{0) = 1},
Qt = {(^.0 • S{t)
1}.
It will be assumed that equations (4.5.27)—(4.5.30) have been suitably dimensionaUzed. In (4.5.29) both curvature effect and kinetic undercooling have been incorporated. Except (3 and /, all other parameters have been taken to be unity. To discuss melting of ice in a supercooled liquid, the transformation f = rT(r,f)-hi,
(4.5.31)
is made which changes the problem (4.5.27)—(4.5.30) to the following problem. ^
= ^ ' ^ ^ « . '
t ( r , 0 ) - t o ( r ) , reQiuQluS[t)-
z = 5,L, ^ > 0 ,
fs{Q,t) = l^ ^ > 0; fL{ro,t)=^fi{t),
(4.5.32)
(4.5.33)
Analysis of Supercooled Stefan Problems with Modified Gibbs-Thomson Relation f^{S{t), t) = -pS{t)S{t), dT dr
i = S,L,
119
(4.5.34)
,5(0-0
= -lS{t)S{t).
(4.5.35)
S{t)+0
If /3 = 0 in (4.5.29), then we have only curvature effect in T(S{t),t). Equation (4.5.35) suggests that we have a Stefan problem with latent heat —lS{t). Now consider the initial and boundary data such that we have a one-phase melting problem for T. We consider the following formulation. fs{0,t) = 1, t>0;
fs{S{t),t)
= 0, t>0]
fiiro. t) = 0, fL = Omre
fs>0,
re [0,1], t > 0,
(1, ro), i > 0.
(4.5.36) (4.5.37)
In terms of Tiir, t), the initial temperature of the liquid is Tiir.O) = - 1 / r , 1 < r < ro.
(4.5.38)
Note that the surface tension which was taken as unity in (4.5.29), is still present. As the initial temperature of the solid is greater than or equal to Tiir, 0), the ice ball with center at r = 0 may dissolve partially in the supercooled water at least for a short time. To preserve the classical one-phase Stefan problem structure, we have T(S{t),t) = 0 which requires that /? ^ 0. The key idea in the existence proof for problem (4.5.36)—(4.5.38) in which /3 = 0 is to consider a sequence of problems for /3n > 0, n == 1, 2 . . . and take the limit as /?„ - ^ 0 . This method of proof is generally employed to develop the proof for the case /? —^ 0. Under suitable assumptions about the data, a unique classical solution (T5, S^) exists with the monotonically decreasing free boundary S^. Some interesting results of [125] are the following: 1. The classical solution of the problem (4.5.27)—(4.5.30) exists and is unique for p > 0 and all time t > 0. 2. If /3 = 0 in (4.5.29), then the classical solution exists and is unique only locally in time. 3. There exists at least one weak solution for /3 = 0 with a monotone free boundary r = S{t) for all time t > 0. 4. Non-existence of the classical solution for the problem with /? = 0 for all time ^ > 0 for certain types of initial data is possible in this one-phase problem. There exists at least one point t = i oi the discontinuity of S{t) such that 0 < 5 ' ( f - 0 ) <S{i + 0), or the continuous function S{t) on a small time interval (0, t*) cannot be absolutely continuous on (£, t*) and it maps some zero-measure set of (f, t^) onto some set of Qf^ ^Qt with a strictly positive measure.
120
Stefan Problem with Supercooling: Classical Formulation and Analysis
It turns out that the results for /^ > 0 are not the same as for P = 0. The kinetic condition acts as a regularizing factor in the dissolution of ice ball in the supercooled water whereas surface tension alone does not have the same effect. The formulation given in (4.5.27)—(4.5.30) was considered in [126] also but with some minor changes in the region under consideration and some change in the boundary condition. We shah continue with the notations used in (4.5.27)—(4.5.35) with the change that S'(O) = So < I and TQ = 1. The boundary condition at r = 1 could be any of the type given below in (4.5.39) and (4.5.40) t ( l , t ) = Q(t), t > 0 ,
df
CT(1, t) = - ^ - ( 1 , t) + T(l, t)-l,t>0. or
(4.5.39)
(4.5.40)
Both (4.5.39) and (4.5.40) are in non-dimensional form. Equation (4.5.40) is a boundary condition of the fifth kind (cf. (1.4.44)) . B(^) is the temperature of the thermostat and C is the specific heat of the thermostat. We shall call this changed problem, Problem (GP). Equation (4.5.40) models the heat exchange between the thermostat and the body. Assume that to(r) e C'[0, S-o] n C'[So, 1] n C[0,1],
(4.5.41)
and the consistency conditions
to(o) = 1, f{So) = 0, ro(i) = e(o),
(4.5.42)
are satisfied. The result stated in Proposition 4.5.2. can also be estabhshed for Problem (GP) (cf. [126]). Asymptotic stability of stationary solutions of Problem (GP) was also studied in [126] and it was shown that under suitable assumptions the problem has one or two or three stationary solutions. One of them corresponds to the case in which the solid phase has melted completely. This solution is always asymptotically stable as ^ —> +00. The other two stationary solutions have non-empty solid regions. The solution with the largest radius for the solid is stable, the other is unstable. A two-phase one-dimensional Stefan problem in the region 0 < x < 1 with a kinetic condition of the form T- = T+ = eS{t), at x = S{t),
(4.5.43)
was considered in [127]. In (4.5.43), 6 is a constant without sign restrictions. Similarly S{t) has no sign restrictions. T'^ is the temperature of the region to the right of .S'(^), and T~ is the temperature of the region on the left of S{t). There are no sign restrictions on T^ and T~ and so superheating or supercooling may exist. S{0) = SQ, 0 < 6*0 < 1. There are no sign restrictions on the initial and boundary data. The main result of this
Analysis of Supercooled Stefan Problems with Modified Gibbs-Thomson Relation
121
study is that introducing a kinetic term at the interface can prevent finite-time blow-up even if supercoohng (superheating) exists. Under suitable assumptions on the initial and boundary data, the existence of a global-in time solution has been proved using Schauder's fixed point theorem. Here global-in-time solution means that either i* = oo and 0 < S{t) < 1, t < oo, 0 < t < t^ or t^ < -hoo and limt^t^_ S{t) = 0 or 1. Uniqueness of the solution has also been proved. The solution of the classical Stefan problem can be retrieved by taking the limit £ —> 0 if correct sign restrictions are imposed on the initial and boundary data and the free boundary is assumed to be nondecreasing.
4.5.4
Multi-dimensional supercooled Stefan problems and problems with the modified Gibbs-Thomson relation
The one-phase supercooled Stefan problem in two and three dimensions without regularization by the Gibbs-Thomson condition has been studied in [128] with particular interest in the mechanism of cusp formation for interfaces that are initially smooth. The problem considered is as follows: Tt = V^T, X e n{t) C R^,t>
0,
(4.5.44)
forx G ^(0),
(4.5.45)
= 0, forx G dn{t), t > 0,
(4.5.46)
V-n = - V T • n, J: G dn{t), t > 0.
(4.5.47)
T(x,0) = To{x)<0, T{x,t)
Here, N = 2,3, dQ{t) is the boundary of Cl{t) which is a free boundary and ft is the outward unit normal on this boundary. As discussed earlier in the one-dimensional case, the equation ct = V'c - x^,,,,
X G /?^, t > 0,
(4.5.48)
where c stands for the concentration, is more suitable to handle a supercooled problem. X is the characteristic function of 0(t) which for each t takes the value unity if X G 0(t) and zero elsewhere. With the help of Baiocchi transformation [129], c{x,t) can be defined as rS{x)
c{x,t) = -
T{x,^)d^,
(4.5.49)
where the free boundary has been written as t = S{x), t > 0, S{x) = 0 for t = 0 and T{x,t) satisfies (4.5.44)-(4.5.47). We have V-(Vc)
=
r
V.{\/T)d^-V{S{x))VT{x,S{x)),
JS{X)
=
/ ^ - ( i ^ + l = T ( x , t ) - h l = Q + l. Js(x) ut
(4.5.50)
122
Stefan Problem with Supercooling: Classical Formulation and Analysis
In obtaining (4.5.50), the relation (4.5.51) has been used which can be derived as follows. Let ^x,t) =t-S{x) = 0. On using (1.4.15) and (1.4.25) in (4.5.47), we obtain V{S{x))VT{x, S{x)) = - 1 .
(4.5.51)
By differentiating (4.5.49), we get Q = T{x,t). In the one-dimensional Stefan problems, to prove the existence and uniqueness of solutions, fixed point arguments combined with classical 'parabohc regularity theory' can be used. In the multi-dimensional Stefan problems, there is lack of regularity in space at the free boundary which does not allow the fixed point arguments to be used. Suppose the free boundary dQ{t) G C"^'" for each t > 0, m (integer) > 0, 0 < a < 1 then T{x, t) would be at most C"^-^ in space by the classical regularity theory and then ^ G C"^-!'-, (4.5.52) on in space. Solving (4.5.47) by 'characteristics' one would obtain atmost dQ{t) G C^~^'^ in a hypothetical fixed point argument. One way to overcome this difficulty is to regularize the supercooled problem by introducing curvature effects at the free boundary and write T{x,t)=
sV{dn{t)),
(4.5.53)
where P is a regular second order operator. By taking the limit £ -^ 0, the problem (4.5.44)—(4.5.47) can be obtained provided enough 'a priori' estimates can be proved. A constructive approach has been adopted in [128] and a method has been described to obtain a short-time solution of the problem transformed in terms of c{x,t). The manner in which cusps develop is described. By using perturbation analysis and matched asymptotic expansions, a mechanism of cusp formation is presented for interfaces that are initially smooth. Even small amount of undercooling can give rise to singularities. A one-phase two-dimensional Stefan problem with kinetic condition has been considered in [130] in the region Q^ C i?^ x R^ where Qt = {{x,y) : 0 < y < g{x, t), x £ R,0 < t < t^}, y = g{x,t) is the equation of the free boundary, g{x,Q) = go{x) > 0, x e R. The free boundary conditions are VT • n = T ; and T = V-n = gt/{l + glY^^.
(4.5.54)
The remaining part of the problem formulation consists of the heat equation and initial and boundary conditions and is given below. Tt = \/^T, {x,y,t)
Ga,
T(x, 2/, 0) = To(x, y) ; T(x, 0, t) - b{x, t).
(4.5.55) (4.5.56)
Under suitable assumptions on the compatibility of initial and boundary conditions at y = 0, and at the initial position of the free boundary and some regularity conditions, the
Analysis of Supercooled Stefan Problems with Modified Gihhs-Thomson Relation
123
local-in-time existence and uniqueness of the classical solution have been proved using Schauder's fixed point theorem. It was observed that the introduction of curvature term in the temperature at the free boundary provides more regularity to the temperature than the kinetic condition alone. A classical two-phase Stefan problem with the modified Gibbs-Thomson relation has been studied in [131] in a region G C J^",n > 2. Let G{t) = G x {t} for \/t € [0,t^],G{t) = Gi{t) U G2{t) U S{t), where Gi{t) is the solid region, G2{t) is the liquid region and S{t) is the phase-change interface separating solid and liquid regions. Let G^* = Uo 0, /3 > 0 on r,
(4.5.57)
T{x, t) = g{x, t) on {G x {t = 0}} U {dG x [0, t,]} ,
(4.5.58)
5(0) = To.
(4.5.59)
Here, n is the unit normal vector pointing from Gi{t) to G2{t). Equations (4.5.57)—(4.5.59) together with heat equations in the solid and the liquid regions and the Stefan condition at the interface completes the formulation of the problem. Thermal conductivities of the solid and the liquid phase are taken unequal. The main result of [131] is given by the following proposition. Proposition 4.5.3.
Let
g e C^{GX [0, t,]) n C'+" ( G I ( 0 ) X [0, t,]) n C'+" ((52(0) X [0, t.]) ,
(4.5.60)
for some fixed a > 0, G{0) = Gi{0) U G2{0) U FQ. Here, FQ is the boundary of an open set with dist(Fo,aG) > 0, and FQ G C^"^".
(4.5.61)
Assume that g and FQ satisfy the compatibility conditions gt{x, 0) - K^ V^g{x, 0) = 0, x G dG n ^G^{0), z = 1, 2,
(4.5.62)
and g{x,0) = -aKc{x)-
(51
on
on
(x,0), X G F O ,
(4.5.63)
where g = gi in G,:(0), z = 1, 2, Kc{x) is the sum of the principal curvatures of FQ. If the above conditions are satisfied, then there exists a time ^0 > 0 (depending on F, a and P) such that the Stefan problem with condition (4.5.57) admits a unique solution (T, F) for t G [0, ^0] and
124
Stefan Problem with Supercooling: Classical Formulation and Analysis
and r e c3+-'(3+-)/2.
(4.5.65)
The key idea in proving the existence and uniqueness of the solution is that when translated to local coordinates, (4.5.57) is a quasi-linear parabohc equation on a manifold without boundary. For a given T{x, t) solve (4.5.57) to obtain the velocity of the interface. This provides us with the interface F which can be used to solve the parabolic problem in the bulk with the Stefan condition on F. A new temperature function T = V(T) is obtained. Using the regularizing character of the operator P , it can be proved that V is a contraction on a small time interval. Neumann condition or mixed conditions can also be prescribed on the fixed boundary dG x [0,to]The local-in-time existence of the solution of a more general two-phase Stefan problem with the modified Gibbs-Thomson relation has been proved in [132]. The parabolic operator in the heat equation is of the form VjTj=
J ^ - V , ( A j ( a : , i ) V j W . = /^ in Gj{t) for V^ G [0,io], J = 1,2,(4.5.66)
and 0 < ^ < ^0 is the time interval in which the solution exists. The domain Gi {t) £ G C R^,n>2 and dGi{t) = T{t) C G.'ite [0, to], is the free surface. G2{t) = G\Gi{t), 0 < t
(4.5.67)
where a > 0, and the unit normal n is pointing into the liquid. The problem formulation can be completed by adding Stefan condition and initial and boundary conditions to the equations (4.5.66) and (4.5.67). To prove the existence of the solution, the problem in time varying domains Gj{t), J = 1,2 is first converted into a problem in fixed domain r2j(0), J = 1, 2 by using 'Hanzawa transformation' [133]. Then a local coordinate system is introduced in a neighborhood of F(0). Under suitable consistency conditions (see [132]) and the regularity of parameters and coefficients, an initial approximation of the solution of an auxiliary problem makes it possible to construct a classical solution of the given Stefan problem on F(0). To apply Newton's method of successive approximations it is necessary to obtain conditions for the invertibility of the 'Frechet derivative' [22] on the initial approximation. Once a regularizer of the Frechet derivative is constructed, the existence of the solution in a very small time interval can be proved with the help of Newton's method. Multi-dimensional one-phase and two-phase Stefan problems with Gibbs-Thomson relation have been discussed in [134] under the assumption that the surface tension e is small. Although classical solutions have not been discussed, some of the observations made in [134] are interesting. Under the assumption that the classical Stefan problem with 5 = 0 has a smooth free boundary and that its classical temperature solution T exists, an approximate solution of the Stefan problem with £ 7^ 0 is taken as T -h £ii (one-phase problem). The existence and uniqueness of the weak solution of the problem
Analysis
of Supercooled
Stefan Problems
with Modified
Gibbs-Tfiomson
Relation
125
formulated for u has been proved. The free boundary of T + eu has also been analysed. In the two-phase problem the perturbations are of the form (Ti -h eui,T2 + £U2) where Ti -h eui is the temperature of water region and T2 + £U2 is the temperature of ice surrounding the water region. Assuming t h a t Gibbs-Thomson relation lowers the equihbrium temperature, the following conclusions have been drawn. In the one-phase problem, small surface tension decreases the water region for all small times and the small surface tension increases the water region for all large times provided the d a t a are radial or close to radial. The results have been extended to general shapes. This observation is radically different in the two-phase problem in which the small surface tension decreases the water region for all times provided the d a t a are radial or close to radial. On the basis of physical arguments, it was mentioned earher in § 4.5.1 t h a t surface tension has stabilizing effect on the growth of planar phase-change boundary. Investigation of stability or instability of perturbations of the planar phase-change boundaries is a vast subject (cf. [135, 136]) and requires rigorous mathematical treatment. We present here some simple results obtained in [137] on the morphological instability of the similarity solution of a one-dimensional Stefan problem in an infinite region. Consider the following two-dimensional one-phase problem of the solidification of a supercooled liquid. Tt = V^T, S{x,y,t)
7^ 0, ^ > to, (x,y)
G R^.
(4.5.68)
On the free boundary S{x, ?/, t) = 0, we have T = -aKc;
and {S/T.VS)
= St, t > to,
(4.5.69)
Kc = dVS'l'V^S' - ^ V{\VS\^).VS)/{2\VSf)
(4.5.70)
T(x, y, to) = To(x, y); S{x, y, to) = So{x, y),
(4.5.71)
To{x) ^ Too G ( - 1 , 0 ) as X -> 00; To{x) ^ 0 as x - ^ - 0 0 .
(4.5.72)
Consider a one-dimensional one-phase problem in the region — 00 < x < 00 in which the region x < 0 is ice at the phase-change temperature T = 0 and the region x > 0 is occupied by the supercooled water at T = Too- A similarity solution of this onedimensional problem can be easily obtained by following the procedure used to obtain the Neumann solution (cf. (1.3.11)—(1.3.13)). Let this solution for a = 0 be denoted by (Ti, 5 i ) , where Ti
= Too + 2aexp{a^){l
- eTi{x/{2y/t))),
for x > 2aVt,
] (4.5.73)
- 0, Si{x,t)
X < 2ay/i
= (x-2aVt.)
= 0,
J (4.5.74)
126
Stefan Problem with Supercooling: Classical Formulation and Analysis
Here, a is the root of the transcendental equation roo
2aexp{a^)
exp{-y^)dy
=-Too-
(4.5.75)
T,(x, y, t) = Ti^ £u{x, y, t) + 0{s^),
(4.5.76)
This similarity solution is perturbed so that
Ss{x,y,t) = Si{x,t)-heR{x,y,t)
+ 0(5^).
(4.5.77)
As the phase-change boundary is no more planar after perturbation, we shall have a two-dimensional problem with the effect of surface tension included in the temperature at the free boundary as in (4.5.69). For hnear stability analysis, 71 and Se are substituted in (4.5.68)—(4.5.71) and the problem is reformulated in terms of u and R. The Fourier transform with respect to y of the equations yields one-dimensional time dependent initial-value problems for different values of the transform parameter whose solutions are analysed for the stability analysis (cf. [137]). The linear stability analysis showed that if the surface tension is non-zero, then each perturbation mode with a non-zero wave number will be stable. However, the solution is unstable with respect to perturbations with a zero wave number limit point in their Fourier spectrum. Concluding Remarks The discussion in §§ 4.1—4.5, is focussed on some modelling and analysis aspects of supercooling. The phenomena of nucleation, conditions at the contact line of manifolds belonging to the fixed boundary of the domain and the free boundary, and the modelling of surface tension and kinetic condition, are quite complicated. For further information references given in [54] are suggested. Surface tension can be taken into account by incorporating surface energy in the energy functional without introducing a sharp phasechange interface [138]. In [139], ideas and methods of the theory of minimal surfaces have been introduced as well as new estimates for time derivatives of non-integer order. Ideas and methods of the geometry of manifolds have been used in [140]. Landau-Ginzburg theory, which is the basis of standard phase-field models, relies on general thermodynamic considerations. In the hmit, Stefan problem with modified Gibbs-Thomson law is obtained. Assuming that the solution of phase-field equations exists, an asymptotic solution of the phase-field equations has been constructed in [141] and the existence of its smooth solution has been proved.
4.5.5
Weak formulation with supercooling and superheating effects
The enthalpy equation (4.2.2) can be written as Cp^
-h / ^ = V • {K{T)VT),
in 17, = Q X (0, t,), Q C 7?^
(4.5.78)
Analysis of Supercooled Stefan Problems with Modified Gibbs-Tfiomson Relation xeH,{T-Tm).
127
(4.5.79)
Here, K{T) - KsT, T < Tm and K{T) - KLT, T > T^ and Hg is the Heaviside graph defined in (4.3.45). x is the hquid fraction present in any phase. In the classical Stefan problem x == 1 in the liquid phase and x = 0 in the solid phase. If the mushy region is present, then x ^ [0, !]• Equation (4.5.78) holds in the distributional sense. We shall now include supercoohng and superheating effects in the weak formulation given by (4.5.78) and (4.5.79). If the interface is not at the equilibrium temperature T^, then the condition T = T^ at the interface in the absence of curvature effects, is replaced by the kinetic condition S(t) =/3{T{S(t),t)),
(4.5.80)
where l3 : R -^ R is continuous and increasing and p{0) = 0. This represents supercooling or superheating effects due to the phase-change velocity (kinetic condition). The standard Stefan problem is then obtained in the hmit as l3'{0) —> +oo (expand (3 in powers of T). For small departures from the equilibrium temperature, the melting or solidification rate is proportional to the departure [42]. When a mushy region is present, i.e., when x ^ [0,1] and X is considered a function of x and t, (4.5.79) should be replaced by a relaxation law for X of the form [101] a^
+ H;\X)
B P(T - T„) in Q„
(4.5.81)
where a is a positive constant, x = xi^^t)^ i^^t) ^ ^u ^nd the term on the r.h.s. of (4.5.81) is an approximation of the term on the r.h.s. of (4.5.80). In (4.5.81), there is no diffusion of x- It has been assumed that the phase transition is much faster than the heat diffusion. In the classical Stefan problem the phase-change is driven by the release or absorption of latent heat at the interface. In the relaxation model (4.5.81), the latent heat released or absorbed is diffused into the system. The relation (4.5.81) accounts for the movement of the interface through the mushy region in the weak sense. As mentioned earlier, in the metallurgical literature concerning solidification of a pure metal, there are two basic modes of solidification, viz., directional solidification and equiaxed solidification. In the directional solidification, which is also called columnar growth, the kinetic law (4.5.80) is considered at the phase-change boundary and the classical formulation is used. In the equiaxed solidification or phase relaxation model, a weak formulation is considered using (4.5.78) and (4.5.79). In casting, at first an equiaxed zone is formed in contact with the wall of the mold. Then a columnar region moves towards the interior, while in the remainder of the hquid, nucleation occurs and an equiaxed solid phase grows, until the two solid phases impinge on and eventually occupy the whole volume (see [142] and a large number of references reported there). In [101], the existence and uniqueness of weak solutions of several Stefan problems with supercooling/superheating effects have been discussed using the theory of non-linear semi-groups of contractions. These weak formulations correspond to: (i) problems of phase relaxation in homogeneous systems (generalized to heterogeneous systems) in which
128
Stefan Problem with Supercooling: Classical Formulation and Analysis
energy equation is written using non-equilibrium thermodynamics, (ii) Stefan problems with phase relaxation, (iii) wave propagation for heat. In [101] and [102] only kinetic condition has been considered. Weak formulation of the Stefan problem with surface tension has been considered in [143]. By coupling the heat equation with the Euler's equation of a non-convex functional (which represents the Gibbs free energy), two weak formulations are given and existence of the solution is proved for each one.
Chapter 5 Superheating due to Volumetric Heat Sources: The Formulation and Analysis Some amount of superheating of the sohd occurs with the melting of pure metals. When superheating is present, a 'mushy region' may develop in front of the phase front, separating the stable solid phase from the stable hquid phase. Note that a superheated solid is in a solid state but with time degenerates into mush. The temperature of the mushy region is taken to be equal to Tm if the phase-change is taking place at Tm- The degree of superheating is small and it is generally neglected in the formulation but when melting takes place due to strong volumetric heat sources, there is clear evidence of the superheating of the solid. The theoretical analysis of a sharp melting front model in the presence of strong volumetric sources indicated 'superheated regions' in the solid just in front of the interface [144]. Superheating or supercooling indicates the instability of the sharp interface and requires corrective measures in the modelling of classical Stefan problems. Some of the corrective measures are: (1) consider an enthalpy model in the place of the classical model, (2) introduce modified Gibbs-Thomson relation in the temperature relation at the phase-change interface in the classical formulation, and (3) consider a different type of formulation such as a 'classical enthalpy formulation' (CEF).
5.1
T h e Classical Enthalpy Formulation of a Onedimensional Problem
We shall first discuss CEF in the context of one-dimensional melting due to volumetric heat sources in a slab ( — 1 < x < 1). This physical problem arises in the 'spot welding' of two large metal sheets of equal thickness of some heat and electric conducting material. Two circular electrodes are placed opposite to each other with the two sheets held tightly in between the electrodes [145]. A high electric current is passed through the system for a short time. The energy produced by the current raises the temperature of the sheets. It 129
130
Superheating due to Volumetric Heat Sources: The Formulation and Analysis
will be assumed that the heating takes place due to Joule heating throughout the volume of the conducting material. If the conditions are symmetrical about x = 0 then it is sufficient to consider heat transfer in only one sheet with no flux condition at x = 0. Under certain assumptions, a one-dimensional problem can be formulated in the region 0 < X < 1 (X isdimensionless). At X = 0, the two sheets are joined and at AT = 1, the electrodes are cooled. According to the maximum principle, the temperature will be maximum at X = 0. The temperature at X = 0 will rise only due to the heat received by it due to Joule heating and not by conduction as other parts are at a lower temperature and because dT/dX = 0 at X = 0 (because of symmetry). Liquid will form at X = 0 when the specific enthalpy at X = 0 becomes CiTm + L It is assumed that ^ = 0 is the time when the temperature at X = 0 rises to the melting temperature T^. For ^ > 0 the specific enthalpy at X = 0 will rise from CsTm to CiTm + / (we assume the specific heat of the mushy region also to be Cs)- During this time the enthalpy of the points in some neighbourhood of X = 0 will have risen. By the time melting starts at X = 0, i.e., the enthalpy at X = 0 rises to CiTm + /, a mushy region will have developed in some neighbourhood oi X = 0. This clearly suggests a three-region formulation, given first in [146], and then extended in [147]. The dimensionless formulation described in [147] is given below. In the solid region f)T f)^T -gf- = 4 ^ + As + BsTs;
As > 0, As + BgTs > 0, 5i(t) < X < 1, 0 < i < i e < o o ,
TsiX, 0) = fPiX), aril)
/^'(O) = 1,
^
0 < X < 1,
.d^f's'' 0,5 a "5" dX^
(5.1.2)
-hAs + Bs>0,
(5.1.3)
x=o
dT< ^ ^s +diTs = d2{t), onX = h
o n X = Si{t); 5i(0) = 0, ^ = S^\X), dTs/dX
(5.1.1)
(5.1.4)
S^^{0) = 0.
(5.1.5)
= 0.
In the dimensionless form T^ = 1 and ^4^, Bs and di are known constants. The equation of the solid-mush boundary is X = Si{t) ov t = Si^{x), t^ is the time at which the mushy region becomes extinct, and a is the dimensionless diffusivity. In the mushy region dt
Q{HM)]
S2{t) <X < Si{t), 0
te,
(5.1.6)
5.1 Classical Enthalpy Formulation of a One-dimensional Problem
HM = 1, a.tt = Q{HM)
131
(5.1.7)
S{\X),
=
As + Bs. HM = 1.
=
Di-h
=
AL + BL.
D2HM, 1
CL/CS
(5.1..
+ A,
+ X.
Here, HM is the enthalpy per unit volume of the mushy region, A = l/{CsTm). Di and D2 are unknown constants. An exact form of Q cannot be guessed. A simple assumption that Q is a linear function of enthalpy in the mushy region has been made in (5.1.8). In the liquid region:
0 < X < S2{t) < Si{t), t > t,,
(5.1.10)
TL \X- S2{t)
2dTL
{CL/Cs)ai
dx
dS2 {{CL/CS) " dt
+ X-HM},
dTL
dx
(5.1.9)
onX
0.
= ^2(0,
t >
t.,
S2{U) = 0, (5.1.11) (5.1.12)
Here, X = S2{t) is the equation of the liquid-mush boundary and t = t^ is the time at which S2{t) starts growing. »S'2(^*) = 0. The above formulation requires some elaboration. The conditions in (5.1.3) have interesting physical interpretations. For example /^ (0) = 1 implies that the temperature at X = 0 is the melting temperature. It is generally understood that if more volumetric heat is supplied, then the enthalpy ai X = 0 should rise. But this does not always happen. For example, if the last condition in (5.1.3) is not satisfied then the temperature at X = 0 for ^ > 0 first becomes less than unity. After some time dT/dt becomes greater than zero so that the temperature starts rising, and again the temperature at X = 0 becomes unity and then the third condition in (5.1.3) is satisfied. ^ = 0 has been taken as the instant of time at which this happens. In view of the boundary condition (5.1.12), we should have the second condition in (5.1.3). As mentioned earlier, the melting does not start at ^ = 0 but at ^ = ^*. This is the time at which the dimensionless enthalpy at X = 0 becomes CL/CS + A. The instant t* can be calculated by obtaining the solution of (5.1.6) subject to (5.1.7) and (5.1.8) (cf. [147]). All the temperatures have been made
132
Superheating due to Volumetric Heat Sources: The Formulation and Analysis
dimensionless by dividing them by the melting temperature Tm^^K). The dimensionless time t is obtained by dividing the real time by td, which is an appropriate time with dimensions and ^ = 0 is time at which all the conditions in (5.1.3) are satisfied for the first time. Till t = t^, there exists only sohd-mush boundary X = Si{t) but for t > t^, we have liquid-mush boundary X = S2{t) also. The assumption of the continuity of enthalpy at X = Si (t) implies CM = Cs which for the sake of simplicity has been assumed to be so in the whole of the mushy region. The condition dTs/dX = 0 at X = Si{t) in (5.1.5) can be derived independently of the assumption of continuity of the enthalpy. The energy balance at X = 'S'i(^) gives
dx\ ' / / M > 1 as X t S{t)- 0 and Hs = I diS X i S{t)+0. Since TM = 1 in the mushy region BTM/OX = 0 as X T S{t)-0. It can be easily concluded from (5.1.13) that dTs/dX > 0. In view of Ts{Si{t), t) = 1 and Ts{X, t) Si{t), dTs/dX te, we shall have a two-phase classical Stefan problem. Initially Si(t) moves very slowly as Si(t) - 0(6-^/') (cf. [147]).
5.2 The Weak Solution
133
For a short-time, asymptotic behaviour of 6*2(i) which depends on Si{t) can also be calculated. When Si{t) ^ Eit^^'^ and S2{y) ~ ^2?/^^^, y = t — t^, V > 0 then E2 is given by E2 = {2£2 -al
+ ^{2Ej - air + 2AEW2 }/4,
(5.1.14)
where Ei is the root of the equation Q2M v ^ ^ i e x p {Ellal)[l
- eii{E,las))
- as ^
4
aX2
7 ( ^ 5 + 55).
(5.1.15)
By taking initial temperature of the sohd as constant, viz., —P several results concerning the solution of CEF were obtained in [148] which include the asymptotic behaviours of Si{t) and 5*2(t), finite-time extinction of the mushy region, and the result that S'i(^e) < S2{te). In CEF, superheating in the solid does not occur. CEF regularises the classical twophase sharp interface problem in the presence of heat sources in which superheating would otherwise occur. Weak formulation also provides regularization. Is there any connection amongst the solution of CEF, weak solution and the classical solution? This question has been analysed in [149] and [150] for a short time by considering a one-dimensional problem in the region — CXD < a; < 00 with the free boundary starting at x = 0. Since superheating, if present, will be restricted to a small neighborhood of a: — 0, it is sufficient to consider the behaviour of the initial temperature f{x). To understand the connection between the classical solution and the weak solution, we should first define a 'weak solution'.
5.2 5.2.1
The Weak Solution The Weak Solution and its Relation to Classical Solution
The definition of the weak solution of a Stefan problem is based on the classical formulation of the Stefan problem. For simplicity, first a weak formulation of a onedimensional two-phase Stefan problem will be defined in the region 0 < x < 1. For multi-dimensional Stefan problems, weak formulations can be defined similarly (see § 11.2.1). In terms of the enthalpy H{T) (per unit volume), the two heat equations in the solid and liquid regions can be written as a single equation as follows:
where K{T) and Q(T) are defined as K
= A'i(T), in the solid, 0 < x < 5'(t),
134
Superheating due to Volumetric Heat Sources: The Formulation and Analysis = Q(T)
K2{T), in the liquid, S{t) < x < 1,
(5.2.2)
= As-\- BsT, in the sohd, = AL + BLT, in the hquid.
(5.2.3)
More general expressions of thermal conductivities and heat generation terms can be considered. Let the phase-change temperature be denoted by T^. Enthalpy H{T) is defined by (4.2.1) (take 0 - 0 for T < T^ and 0 = 2 for T > T^). By inverting (4.2.1), we obtain T = 13(H),
(5.2.4)
where PiH)
=
H/ipsCs),
= =
H < psCsTr^,
0, psCsT,„
- 1/CL,
PLCLT^
+ PLI,
H > PLCLT^
(5.2.5)
+ pil. J
Equation (5.2.1) should be supplemented with the initial and boundary conditions such T{x,0) = To(a;), 0 < x < 1; T(0, t) = gi{t) < 0, T(l, t) = g2{t) > 0,
(5.2.6)
Equation (5.2.1) is satisfied in the distributional sense as dH/dt does not exist at T = Tm in the classical sense. The classical formulation of the above two-phase Stefan problem is simple and will not be given here. Multiply (5.2.1) by a suitable test function ^(x, t) G C2'i([0,l] X [0,^*]) such that V^(0,i) = V^(1,0 -= 0, 0 < ^ < ^*, and V^(x,^*) = 0 , in 0 < x < 1,
(5.2.7)
and integrate over the (a:, t) region. We obtain JQ
JO
'^IHf)+^^-^^
dt
dxdt = 0.
(5.2.8)
On integrating by parts the first term in the integrand in (5.2.8), we obtain
Jo
IIJK
dt
dx
If
Jo Jo
K — ^ dxdt + ipQ dxdt dx dx Jo Jo
- f\ijH]i*dx+
(5.2.9) [^ t* H^dxdt =0 Jo Jo Jo ot The first term on the l.h.s. of (5.2.9) in zero in view of (5.2.7). Integrating the second term in (5.2.9) by parts, we obtain
- J
ox
dt+ [ r T— (K-;^ I dxdt + f f* ijQdxdt Jo Jo ox \ ox J Jo Jo
Jo
+
[^ H(ToWx, 0)dx + f^ t
Jo
Jo Jo
H-^dxdt = 0 ot
(5.2.10)
5.2 The Weak Solution
135
On using (5.2.6) in the first term in (5.2.10), we get
- f^' gi{t)K{g,{t))^{0,
t)dt - J^ H{To)^Pix, 0)dx.
(5.2.11)
If a pair of bounded measurable functions (T, H) exists such that (5.2.11) is satisfied for all test functions ip{x,t) satisfying (5.2.7), then the pair {T,H) is called a weak solution of the classical two-phase one-dimensional Stefan problem in the region 0 < x < 1, 0 < t < t^:. Note that in the definition of a weak solution, the boundary conditions at the interface x = S{t) are not required. So they do not appear in (5.2.1)—(5.2.6). A weak solution for multi-dimensional two-phase classical Stefan problem can be defined similarly. In this case integration by parts is to be done with the help of Green's formula. Other types of boundary conditions can also be considered in (5.2.6) and in that case test functions have to satisfy appropriate boundary conditions. The boundary conditions for test functions are chosen in such a way that all those terms appearing in the integration by parts, which are to be evaluated at the boundary and which are not prescribed, are eliminated from the equation (5.2.11) through which the weak solution is defined. For example, in (5.2.9) the boundary conditions ip{0,t) = '0(1, t) = 0 eliminate the term [dT/dx]l as these boundary conditions are not prescribed in the present problem. Similarly weak formulations of more comphcated problems and their weak solutions can be defined. A classical solution is a weak solution. This proposition holds for a general Stefan problem but it will be discussed here for a simple one-dimensional problem given in (5.2.1)—(5.2.6). To avoid repetition, we give here only outlines (see (5.2.12)—(5.2.14) for details). If a classical solution exists, then we know that (5.2.1) holds separately in solid and liquid regions. T is continuous in 0 < a: < 1, and a bounded measurable function H defined in (5.2.1) also exists. We have to show that the pair (T, H) satisfies (5.2.11). To prove this, we start with (5.2.8) and write the double integral as the sum of two integrals, Ii + I2. In /i the hmits of integration for the x-variable are taken from 0 to S{t) — 0 (solid region) and in I2 the limits for x are taken from 0 to S(t) -h 0 (liquid region) (the limits for t remain the same). The procedure given in (5.2.12)—(5.2.14) is to be followed for both the integrals. On adding the two integrals, we get (5.2.11) with an additional term which is the last term in (5.2.13). The fine integral along x = S{t) in (5.2.13) is zero because the classical solution exists and Stefan condition is satisfied, /i -h /2 = 0 as both 11 and I2 are zero, the remaining terms give (5.2.11). In the weak formulation, the phases are 'pointwise' characterized by enthalpy, whereas, in the classical formulation the phases are 'globally' determined by the interface. Therefore a weak solution of the Stefan problem will be a classical solution provided: (i) initially no mushy region exists, (ii) distributed heat sources (sinks) are absent as in their presence mushy region may develop.
136
Superheating due to Volumetric Heat Sources: The Formulation and Analysis
(iii) a smooth surface exists which divides the region under consideration into two disjoint regions, in each of them parabohc heat equation are satisfied in the sense of § 1.4.6. Temperature is less than or equal to the melting temperature in one of the regions (solid region) and in another region temperature is greater than or equal to the melting temperature (liquid region) with continuity of temperature across the interface, (iv) the Stefan condition is satisfied at the interface. These assumptions are not rigorous, but they serve our present purpose, which is to show that the Stefan condition can be recovered from the weak solution. We shall continue with the formulation given in (5.2.1)—(5.2.7). Assume that a weak solution satisfying the initial and boundary conditions (5.2.6) and equation (5.2.11) exists. A smooth interface x = S{t) exists. Let the region Q{x,t) = {{x,t) : 0 < x < 1, 0 < t < t^} be divided by the curve x = S{t) into two disjoint parts Qi{x,t) and n2{x,t). Qi lies to the left oi X = S{t) and O2 fies to the right. T = 0 on a; = S{t) ( under suitable assumptions on the data this and some other assumptions can be proved also, see § 11.1), ni{x,t) = {{x,t) : 0 < X < S{t)-0, 0
d_ K{T) dT dx dx
If
Jo Jo
•V^-
dt
(5.2.12)
dxdt = /i.
If integrations are performed in (5.2.12) using integration by parts and the Stake's Theorem is applied, then we arrive at the following equation.
+ y^
Jx=Sit)-0
H{Tomx,0)dx
[\
+ j ^ g,{t)K(g,)^{0,t)
.dT_ dt + H^j dx dx
dt
(5.2.13)
On using Stoke's theorem [82], the area integral is converted to the boundary integral and for this we use the relation ipdf/dx = d{ipf)/dx - f dip/dx in (5.2.12). On integrating (5.2.1) with Q{T) = 0 again over the region ^2(0 (call this integral I2) we obtain an equation similar to (5.2.13) (with appropriate changes) but the curve X = S{t) is traversed in a direction opposite to that in (5.2.13). Adding the two equations of the type (5.2.13), one each for Qi{t) and 1^2(0 ^^^ subtracting the sum from (5.2.11) {Q{T) = 0) and remembering that Ii -h h = 0 as (5.2.1) holds, we obtain
L
S{t)
^ dx
dt
[H]dx\
= 0
(5.2.14)
5.2 The Weak Solution
137
Here, [f] means the jump in the quantity / across x = S{t) as we move in the positive x-direction. Since (5.2.14) holds for any arbitrary test function ip and at an arbitrary time, we have dt
ox
/[H], x = S{t).
(5.2.15)
Equation (5.2.15) is the Stefan condition. A similar procedure can be followed for multidimensional problems also. For a solution of the Stefan problem to be a classical solution, in addition to the regularity of the temperature and the phase-change boundary, the temperature of the solid region should be less than the melting temperature and the temperature of the liquid region should be greater than the melting temperature.
5.2.2
Structure of the mushy region in the presence of heat sources
In the presence of heat sources or if melting temperature is a function of x and t, or if mushy region exists initially, a weak solution (WS) will not be a classical solution (CSS). However, if the solution of CEF denoted by CES exists and WS is sufficiently regular, then WS = CES but WS ^ CSS. This is because in CEF, a mushy region is present initially. By considering different cases in terms of the sign of the initial temperature /(x), and the derivatives of f{x) at x = 0, CES, WS and CSS have been compared in [149] and [150]. The phase-change starts at x = 0 in the region —oo < x < oo. The existence of solutions has also been discussed. We present only one result here which is valid only for a short time. If f{x) > 0 for X < 0 and f{x) < 0 for x > 0, h{0) = 0 and nO+) - ^
< 0, then CSS = WS
/ ( 0 + ) = 0, and Ksf"{0+) -h Q = 0,
(5.2.16) nO+) - ^
> 0, then CSS ^ WS
where Q is the constant heat source in — oo < x < oo, and (3 ^ S{0) = -KLh'{0-)/l.
(5.2.17)
By considering a one-dimensional problem in 0 < x < 1, it has been shown in [151] that if the melting temperature is space-dependent, then a mushy region can appear spontaneously even in the absence of volumetric heat sources. The following weak formulation has been considered in [152]. Ht = ({H)U + f{H),
(x, t) e (0,1) X 7J+,
(5.2.18)
138
Superheating due to Volumetric Heat Sources: The Formulation and Analysis (t>{H){0,t) = {(t>{H)Ul,t) = 0,te H{x, 0) - Ho{x), X e [0,1].
R+,
(5.2.19) (5.2.20)
Here, (p{H) can be identified with the temperature and H with the enthalpy per unit volume. /, 0 and HQ satisfy some assumptions (cf. [152]). The source term f{H) and the initial enthalpy Ho{x) are so chosen that enthalpy is increasing in x and t. Initially the region [0,1] is solid. At a; == 1 it passes to mush in time and then to hquid. Interfaces appear first between solid and mush and then between mush and liquid. These recede monotonically towards x = 0. Under suitable assumptions on (/), /, HQ it has been shown that solid-mush and liquid-mush interfaces are continuous, enthalpy is continuous across the solid-mush boundary but not across the mush-liquid interface and the mushy region disappears in finite time. In equation (5.1.13) it has been assumed that the mush is progressing on the sohd. If the solid is progressing on mush, then it cannot be proved that the enthalpy is continuous at the solid-mush boundary as the Stefan condition (5.1.13) with a positive sign on the r.h.s. is to be considered. If S changes sign at both sohd-mush and mush-liquid boundaries, then the regularity of the free boundary and of the velocity satisfying the Stefan condition has been examined in [153]. The enthalpy has been assumed to be an increasing function of x. An example has been constructed in [154] in which the velocity of the free boundary changes sign infinitely many times. Classical enthalpy formulation arises in thermal switching problems also (cf. [155, 156]). Certain materials such as some types of glasses and vanadium dioxide, change phase at relatively low temperature, such as, at 70°C. In the hquid state the electrical conductivity of such materials increases by a factor of 10"^. For such materials, in Joule heating, the heat source is given by a{H)E'^, where a is the electrical conductivity, H is the enthalpy and E is the electrical field which depends on a and some other parameters. In essence, heat source may depend on the free boundary, and the latent heat / depends on the history of the free boundary [156]. The existence of the solution to a one-dimensional classical enthalpy formulation of the thermal switching problem have been proved in [156]. The emphasis is on the solution for the solid region and the solid-mush boundary. The non-homogeneous term in the heat equation contains the free boundary due to the dependence of the heat source on it. The behaviour of mushy regions in spacial dimension, n > 1, under the action of volumetric heat source depending on, x, t, and enthalpy, has been studied in [157]. Conditions on the heat source necessary for the appearance of a mushy region inside a purely liquid or a purely solid phase have been discussed. As far as regular solutions are concerned, the behaviour of the solution at the free boundary separating a mushy region from a pure phase is related to the behaviour of the source on the free boundary. Under some Lipschitz-continuity conditions on heat sources, a mushy region can expand into a pure phase only if the heat source is non-zero and is of a suitable sign at the interface. Hence, both energy and heat flux are continuous across the interface, unlike the case when a pure phase expands into a mushy region. An example has been given in which a mushy
5.3 Blow-up and Regularization
139
region in a pure phase disappears and again reappears after some time. Both weak and classical solutions have been discussed. For some results pertaining to weak solutions of this problem, see § 11.2.1. By considering a one-dimensional two-phase classical Stefan melting problem in the region 0 < x < 1 with volumetric heat sources and writing its solution in terms of Green's functions, it was shown in [144] that there exists a region S{t) < x < S{t) + d, for some d > 0 in which the solid is superheated. This happens provided the heat source is large and the prescribed heat flux on the solid at x = 1 is small. Linear stability analysis of the classical solution of this one-dimensional two-phase Stefan problem was also carried out for two-dimensional perturbations (three-dimensional perturbations can also be considered). If the heat flux towards the free boundary from the superheated solid side is greater than from the liquid side, then this situation suggests instability. Superheating occurs generally for very pure metals. Its occurrence suggests that for such problems we should have a mathematical model different from the classical twophase formulation as in the classical two-phase model, the solid cannot be superheated. One of the ways to present the superheating of the solid is to introduce the mushy region in the formulation as done in CEF. Another way is to consider enthalpy formulation which is a weak formulation. A grain or dendrite model has been proposed in [158] to describe the microstructure of the mushy region and discuss its stabihty. The solid is considered in the form of a sequence of uniform one-dimensional grains of very small length which are part solid and part liquid in the mushy region. An explicit solution for the one-dimensional version of the model is obtained. It has different forms near the pure solid boundary, in the mush, and near the pure liquid boundary. An appropriate average of this solution for the microstructure formulation, reduces to the weak solution proposed in [145]. The simple microscopic model considered in [158] for the mushy region has been further extended in [159] by including modified Gibbs-Thomson relation that results from the curvature of the interface and the kinetic condition (the normal velocity of the interface is taken away from the liquid). The fine structure of the mush consists of regularly spaced nucleation sites (grains) in one-dimension and a lattice of squares in two-dimension. A method of multiple scales is employed and a classical formulation of the free boundary problem has been used to model the evolution of the two-phase microstructure. Then a microscopic model for the mush is obtained by an averaging procedure. Emphasis is not put on volumetric heating in [159].
5.3
Blow-up and Regularization
As mentioned earlier, a blow-up may occur during the solidification of supercooled liquid if regularization is not done. In the superheating of the solid also, in.the absence of regularization, i.e., non-inclusion of surface tension effect and/or kinetic condition at the interface or absence of mushy region in the formulation, we can raise the three questions
140
Superheating due to Volumetric Heat Sources: The Formulation and Analysis
(A), (B) and (C) (§ 4.4.1) that arose in the context of supercooUng. A heat source term of the form T^+^, a > 0, has been considered in [160] in the one-phase melting Stefan problem with the superheated solid occupying the region 0 < X < S{t), S{0) = So>0. The initial temperature of the solid is To{x) > 0, x e [0, ^o], 0 is the phase-change temperature, and T{0,t) = 0, 0 < t < t^. The temperature To{x) satisfies some assumptions such as: (1) To(0) = To(5'o) = 0, (2) To,a: < 0 on (XQ^SQ) for some XQ G (0, So), (3) TQ^^^ + TQ'^^ > 0 for X G (0, So). The formulation of this problem can be easily written. Let {t^, T{x,t), S{t)) be the classical solution. The main result of [160] can be stated as follows. Proposition 5.3.1. If t^ < CXD, then one (and only one) of the following will always happen. (I) S{t) -^ Soo ^ (0, oo) ast ^ t^ and there exists only one blow-up point x* G (0, 6*00)• (II) S{t) -> -hoo as ^ T ^* and the set {T{x, t)} is bounded on {(^, x) : 0 < ^ < ^*, 0 < X < min (a, S{t)) for each a > 0. It was pointed out earher that questions (A), (B) and (C) (see § 4.4.1) can be answered for the supercooled problem (3.1.8)—(3.1.12) with the help of the quantity Q defined by (4.4.6). A superheated one-dimensional problem can be formulated on the same lines as the formulation of the supercooled problem (3.1.8)—(3.1.12) and so complete formulation will not be given here (cf. [161]). But, for the sake of clarity, it may be mentioned here that the region 0 < x < S{t), S(0) = 1, is the superheated solid and x > S{t) is the liquid region with T^ = 0 • S{t) is progressing towards x = 0. For simplicity we take dT/dx = 0 at X = 0. The quantity PQ given by Po=
/ \ T O ( X ) - 1) c^x,
(5.3.21)
plays an important role in answering the questions concerning blow-up in superheating. To(x) > 0 is the initial temperature of the sohd, 0 < x < 1, and the latent heat of fusion has been taken as unity in (5.3.21) after scahng. In melting, latent heat is negative and Po is the energy required for the complete melting of the superheated solid 0 < x < 1. If Po > 0, then rS{t)
/
{T{x,t)-l)dx
(5.3.22)
will remain positive. The statements (Al), (Bl) and (CI) given below hold. (Al) If the solution exists for all time, S{t) ^ Soo > 0 and T(x, t) ^ 0 as t ^ 00 then - JQ°° dx is negative. (Bl) If there exists some finite time t = t^ such that the solution exists for 0 < ^ < /* and as t ^ t^, S(t) -^ 0 and dS/dt ^ -00, then Po = 0. (CI) If the solution exists only for 0 < ^ < ^* but as ^ -^ t^, dS/dt -^ - 0 0 and S -^ S^ > 0 then the solution blows-up. In the supercooled problem, roughly speaking, when the free boundary touches the negativity set {T(x,t) < —1} then blow-up occurs.
5.3 Blow-up and Regularization
141
In the superheating problem, if the free boundary touches the positivity set {T(x, t) > 1} then blow-up occurs. The heat in the solid near the free boundary is too much and the advancing free boundary cannot remove the heat and bring down the temperature to the equilibrium level which is taken as zero. It is possible to give more general sufficient conditions for a blow-up. If there exists a function f{x) defined for 0 < x < S{0) such that: /(O) > 0, f (0) > 0, f'{x) > 0 for 0 < X < 5(0) and 1
f{To{x) - l)fdx > 0,
(5.3.23)
0
then blow-up occurs. The questions (A), (B) and (C) ( § 4.4.1) were discussed in [161] for a two-dimensional one-phase melting of a superheated solid also. If there exists some /(x), x £ B? with / > 0 and V V > 0 in n(0) = ^o C i?^ VL{t) = Qx {t}, Q C R'^, 0
i
/(To -l)dx>
0,
(5.3.24)
no
then case (C) must occur. If case (B) is to occur, then the integral in (5.3.24) should be zero for all the harmonic functions / . If / (To — l)c/x = 0, but there is some harmonic no function / such that / /(To — 1) dx ^ 0^ then blow-up occurs. dQo
Chapter 6 Steady-State and Degenerate Classical Stefan Problems 6.1
Some Steady-State Stefan Problems
Steady-state free boundary problems occur more frequently in Stefan-like problems such as seepage through dam and free surface flows (cf. (1.1.5)—(1.1.10)). Only a few studies on steady-state Stefan problems have been reported. These problems can be studied from the point of view of the conditions of their origin, and the existence, uniqueness, and regularity of their solutions. In [162] some conditions which lead to steady-state solutions have been discussed. Consider a bounded region Q C R^, n > 1, with a sufficiently regular boundary ^Q = Fi U r2 U Fa, 0 = r^i U r^2 U 5 where Qi is the solid region, ^2 is the liquid region and S is the sharp interface separating these regions. The portion Fi of the boundary dQ is kept at temperature T = 6 > 0, heat flux is prescribed on F2 and the boundary F3 is insulated. A volumetric heat sink g{x), x e Q, per unit volume is acting in O. A steady-state will be reached if the outflow of heat through F2 is large, and g is small. The temperature T{x) in Q is defined in the following way. T{x,t)
=
Ti{x) < 0 , if x G Qi, (6.1.1)
= 0, li X e S, =
T2{x) > 0, if :r G O2.
The formulation of the steady-state problem consists of the following equations. V^T, - - ^ ,
in 0 „
2 = 1,2; 0 < c/< 00,
Ti = T2 = 0, Ki-^ = K2^] onx on on 142
e S,
(6.1.2) (6.1.3)
6.2 Degenerate Stefan Problems
143
T2\r,=b>0;
and
dT —
(6.1.4) rs
-Kr
-K,
= q if T2 > 0; ^ > 0,
dn
m dn
=
(6.1.5)
9 if Ti > 0,
r2
where, n is the unit normal vector on S pointing into the Uquid. If e = K2T^ -KiT-,
(6.1.6)
where T^ is the positive part of T, and T" is the negative part of T, then we have the following problem to be solved. \/'0 = -g
i n ^ , ^Ir, - = ^ ,
Bf) " ^ r2
de_ dn
0,
(6.1.7)
rs
where B = K2b > 0. The main result in [162] is concerned with the existence of a critical flux q = qc{B,g) such that: (i) {q,g) with q < qc{B,g) implies T > 0 in Q. This means that only one-phase wih be present in the steady-state, (ii) {q, g) with q > qdB, g) implies T has both negative and positive values in Q. This in turn imphes that both solid and liquid phases will be present in the steady-state. First, (6.1.7) is formulated as a variational problem. Then with the help of the properties of the solution of the variational problem, qc has been obtained. Some steadystate problems have been considered whose explicit solutions verify the theoretical results. The problem considered in [162] is a generalization of the problem in [163] in which g = 0. It has been proved in [163] that there exists a g^i > 0 such that for all q > qi, we have a steady-state two-phase problem in r^, and K2b qi
• meas (r2),
(6.1.8)
tto
where a^ is some constant having dimensions of (length)"
6.2
Degenerate Stefan Problems
The term degenerate Stefan problem is used for a Stefan problem in which the mathematical nature of the differential equation or the boundary condition changes if some parameter associated with the problem varies within its admissible range of values. Some of the commonly studied degenerate Stefan problems are:
144
Steady-State and Degenerate Classical Stefan Problems
(i) Quasi steady-state Stefan problems or quasi-static Stefan problems in which the heat equation is elhptic but the free boundary is time dependent. Such problems may arise if the the specific heat C = 0 or the temperature has attained a steadystate. The Hele-Shaw problem described in (3.3.45)—(3.3.46) is a quasi steady-state degenerate Stefan problem. (ii) Parabolic-elliptic Stefan problems in which the specific heat C = C{T) is such that C{T)
= > 0, if T > 0, (6.2.9) =
0, if T = 0.
Quasi steady-state Stefan problems Using the theory of conformal mappings, a two-dimensional quasi-static moving boundary problem can be described by a non-linear Lowner-Kufarev equation [164] and a functional relation T between the shape of the free boundary and its velocity can be established. Together with the initial data this leads to an initial-value problem. Assuming that T satisfies certain conditions, the existence of a local-in-time solution of this initial value problem has been proved in [165]. This method is mainly applicable to those free boundary problems in which the domain is shrinking. Continuity conditions on !F are also not easy to satisfy. The proof is based on the convergence of Picard iterative method. In the Stefan problem considered in [166], the free boundary conditions are T = 0, and R{x)Vn = {ut, A{x,t)V:,T)]
on r(^).
(6.2.10)
Here, r(^) is the free boundary, T{t) n ^Q = 0 and dVt{t) = r{t) UdQ^^t e [0, ^*], n is the unit normal outward to r(t), Q{t) = Q x {t}, 0l. A{x,t) is a uniformly elliptic matrix associated with the parabolic operator V defined as VT=
(~^dt
V,{A{x,t)V:,)]
T = F in n{t), 0 < t < t,.
(6.2.11)
Suppose R(x) = 0, X e r(0), R{x) > 0 in Q(0),
(6.2.12)
and dR/dXn < 0 in some neighbourhood A^o of r(0) in Q. Let (An(x), uj{x)) be the local coordinates in A^o, with uj{x) the local coordinates of the projection P{x) of a point X e No on r(0) and Xn(x) the distance from x to P{x). For the conditions in (6.2.12), we have a degenerate Stefan problem. If R{x) = 0 then we have the oxygen-diffusion problem in R'\ Generally the existence, uniqueness and regularity of solution of degenerate Stefan problems are discussed in the context of their weak solutions [167, 168, 169, 170, 171, 172], but the weak solutions are not sufficiently regular to be called classical solutions. In some cases the existence of the classical solutions of deg(nierate Stefan problems has been investigated which are discussed below.
6.2 Degenerate Stefan Problems
145
Using Hanzawa transformation [133] so as to consider a problem in a fixed domain and an analogue of Moser-Nash theorem [173] and assuming some compatibility and other conditions, the existence of the unique solution of the degenerate problem (6.2.10)—(6.2.12) and the solution of oxygen-diffusion problem for R{x) = 0 has been estabhshed in [166] on a suflB.ciently small time scale. Degenerate parabolic-elliptic problems The parabolic-elliptic Stefan problems have been analysed mostly for their weak solutions. Only few studies have been reported on the analysis of the classical solutions of these problems. The regularity of the classical solution of the two-phase one-dimensional degenerate Stefan problem described in (6.2.13)—(6.2.17) has been discussed in [174]. Let the regions — 1 < x < S{t) and S{t) < x < 1 be denoted in the following equations by superscripts 1 and 2, respectively. p\T)Tl
- Ti, = 0, -l<x<
S{t), 0 < t < t „
P\T)T,^ - Tl = 0, S{t) <x
(6.2.14)
T'^\x, 0) = T^^\x, 0), S{0) = 0, T\-l,t)
= g\t)<0,
T\l,t)
(6.2.13)
(6.2.15)
= g^{t) >0, 0
T'^^{S{t),t) = 0; S{t) = T^{S{t)-0,t)-T^{S{t)+0,t).
(6.2.16) (6.2.17)
The basic assumptions are: (Pi) /?^'2 e C^iR^),
P^^HT) > 0 and /?i'2(T) = 0 if and only if T = 0,
{P2) ^xTo'^(O) > 0, =F^^'^(^) > (5 > 0 and To'^(x) x>0 and only if x = 0.
where the inequality holds if
The main result of [174] is the following proposition. Proposition 6.2.1. hold good.
Under the assumptions (Pi) and (P2), the following results
(1) If the data To'^(x) G C ^ [ T 1 , 0 ] and g^^'^{t) 6 C^lO.t^] satisfy second order compatibihty conditions, then the unique weak solution has the following regularity S{t) e C^(0,^.]; T'^^x.t)
e C^ii^l,
S{t)] x [0,^*]).
(6.2.18)
(2) If the data To'^(x) G C ° ° [ T 1 , 0 ] and g^^'^{t) G C°°[0,^*] satisfy the C°° compatibility conditions, then the unique weak solution is also C°°-smooth up to the boundaries t — 0 and X = ±1.
146
Steady-State and Degenerate Classical Stefan Problems
Definition 6.2.1. : C°°-compatibility conditions are called m-order compatible at (x, t) = (0,0) and (=Fl, 0) ifTj'^(x) e C2^[q=l,0], g^^^{t) e C^[0,^*] and there exist functions S'W ^ ^^^[O,^*] and f i'2(a;,^) G C2^'^[5(^), ^1] x [0,^*] such that f i'2(^l, t) = g^^^(t), f'^\S{t),
t) = 0,
(6.2.19)
/ > , t) = /?^'2(f i'2)f/'' - f^f - 0 ( r ) ,
(6.2.20)
5(0) - 0 , g{t) = S'{t)-t^{S{t)-0,t) + t^{Sit)+0,t) = 0(r).
(6.2.21)
If To'^(x) € C ° ° [ T 1 , 0 ] , ^i'2(t) e C°°[0,t,] and (6.2.19)-(6.2.21) are satisfied for any m, then the data are called C°°-compatible. A weak variational formulation of the multi-dimensional degenerate parabolic-elliptic Stefan problem has been presented in [167]. By applying the parabolic regularization technique, the existence, uniqueness and stability of the solution with respect to the data, have been analysed. Boundary control aspects have also been discussed. By considering the weak formulation of a two-phase degenerate Stefan problem, the Lipschitz continuity of the free boundary (under suitable assumptions) has been proved in [169] in some small time interval. These results have been further extended in [170] and under suitable assumptions the free boundary is Lipschitz continuous and temperatures satisfy (6.2.13) and (6.2.14) classically
6.2.1
A Quasi-static Stefan problem and its relation t o t h e Hele-Shaw problem
A one-phase quasi-static Stefan problem (QSSP) in the region Q{t) = Q x {t}, Q C jR", n > 2 can be formulated by taking the specific heat C = 0 in the heat equation in the one-phase Stefan problem. Temperature is static but the free boundary is time dependent. Let dQ{t) be the free boundary which is also the outer boundary of Q{t). Q{t) could be expanding or shrinking for ^ > 0. Shrinking region problems or suction problems, are generally ill-posed. The Hele-Shaw problem (HSP) in R'^ given in (3.3.45)-(3.3.46) can be identified with a Stefan problem if the pressure p of the fluid is regarded as the temperature of the hquid in QSSP. If p > 0, then p is the temperature of the warm liquid which is expanding on ice. If p < 0, then p is the temperature of the supercooled liquid, and ice is expanding on this supercooled hquid. The pressure of the liquid can be increased (decreased) by injecting (withdrawing) fluid though the inner fixed boundary of n(0) if Q{0) is considered to be a doubly-connected region. In this case the fluid is surrounding a region G C R^\ n > 2 and the inner static boundary of 0(0) is dG. The boundary conditions on dG could be dp -— = - Q , Q > 0 (pressure increases), on
(6.2.22)
6.2 Degenerate Stefan Problems
147
dp -— =^ Q, Q > 0 (pressure decreases), on
(6.2.23)
where, n is the unit outward normal to dG. Fluid can also be injected (withdrawn) though point sources (sinks) or distributed sources (sinks) situated in 0(0). Q{t) could be a simply connected region with outer boundary as the free boundary. When sources or sinks are present, the governing differential equation will have singularities. If the two-dimensional Hele - Shaw cell has porous plates though which a uniform suction is applied, then the formulation for pressure can be written as V^p -h F{t) = 0, F > 0, X G n{t); p = 0 and dp/dn = V, on dn{t).
(6.2.24)
Here, V is the outward normal component of the velocity of the free boundary. By using the transformation p = p + F{t) [/,
(6.2.25)
where U is the solution of the problem V^U = 1 in Q{t); f/ = 0 and dU/dn = 0 on dn{t),
(6.2.26)
this uniform suction problem can be converted to a standard Hele-Shaw problem except at the singularities of F. For a two-dimensional HSP, complex variable methods can be used to find some exact analytical solutions. For example, it has been shown in [161] and [175] that due to suction from a point sink, a limacon can become a cardioid with a cusp. For Hele-Shaw problems with shrinking regions a variety of cusp may occur. For both expanding and contracting one-phase Stefan problems, the free boundary has been shown in [176] to depend continuously and monotonically on the specific heat C taken to be greater than or equal to zero. In some cases, temperature has also continuous dependence on C (specific heat). In particular, taking C —> 0, the free boundary in the Stefan problem approaches that of the Hele-Shaw problem and it follows that even wellposed Stefan problems can have free boundaries which can get arbitrarily close to forming cusps. If the free boundary in an expanding Hele-Shaw problem has cusps as ^ -^ oo, then Stefan problem in the hmit t ^ oo, also develops cusps [177]. For n > 2, Hele-Shaw suction problem with boundary conditions (6.2.22) and (6.2.23) and one-phase supercooled water problem were discussed in [177]. Existence of weak solutions for both the problems can be proved if and only if the initial domain belongs to a certain class of domains. Uniqueness does not hold in general. In the ill-posed Heleshaw suction problem, fingering configuration can arise from a suitable initial domain whose boundary is smooth and nearly spherical. In the supercooled water problem the initial domain should belong to a certain class which depends on the initial temperature. Regularity of the free boundary has also been discussed.
Chapter 7 Elliptic and Parabolic Variational Inequalities 7.1
Introduction
Rigorous definitions of elliptic and parabolic variational inequalities will be given a little later. Before that we ask a question, What are the essential features of a variational inequality formulation? Let us consider a simple problem of finding the point XQ G (a, b) at which the unique minimum of a real valued function f{x) G C^[a, b] exists. If f{x) is a convex function, then XQ can be obtained by solving the equation f'{xo) = 0. In general for XQ G [a, 6], three cases arise: 1. If minimum is attained at XQ = a, then f'{xo) > 0. 2. If minimum is attained at XQ = b, then f'{xo) < 0. 3. If xo is an interior point, then f'{xo) = 0. These three conditions can be expressed in terms of a single inequality f{xo){x - xo) > 0, V X G [a, b].
(7.1.1)
Equation (7.1.1) is an example of a variational inequality whose characteristic features are that it is an inequality and it is satisfied for all x varying over the interval [a, b]. We shall now generalize this notion and consider a space of functions, say, H^{Q), Q C K^, n > 1 is an open bounded set and let / be a functional on //^(Q). Find a function u = UQ e H^{Q) or belonging to a subset of H^(Q) such that f{uo) is minimum as u varies over H^{Q) or over a subset of H^{Q). Immediately several questions arise such as: What sort of function spaces should be considered so that a solution can be found? What should be the form of the functional defined on these spaces? Are there equivalent formulations? Answers to these require sophisticated functional analysis tools 148
7.2. Elliptic Variational Inequality
149
whose detailed description is beyond the scope of this book. In the next few sections an attempt will be made to answer some of the above questions in simple mathematical terms. After discussing the theoretical background of ehiptic and parabolic variational inequalities, the formulations of some classical Stefan problems as variational inequalities have been given. It may be noted that for any given classical Stefan problem it may not be possible to formulate it as a variational inequality problem. Weak formulations of Stefan problems, which are continuum models, are easily amenable to variational inequality formulations as variational inequalities are themselves continuum models but this is not the case with classical formulations. To make this volume self-contained, some relevant definitions and theorems are given in the Appendices A—D. The elliptic variational inequalities will be discussed first. Whenever it is possible to formulate transient Stefan problems as variational inequalities, they are formulated as parabolic variational inequalities. Elliptic variational inequalities, which are concerned with elliptic or steady-state free boundary problems, serve as a good starting point for discussing parabolic variational inequalities. This is because many of the ideas involved and approaches used in proving results in the analysis of elliptic variational inequalities can be extended, with appropriate changes, to the analysis of parabolic variational inequalities. This does not mean that every elhptic problem can be extended to a time dependent parabolic problem.
7.2
The Elliptic Variational Inequality
7.2.1
Definition and the basic function spaces
Let r^ be a bounded open subset of R^ with smooth boundary dQ. An inequality of the form a{u,v -u)
> {q,v-u),
\/ v ^ M; u e M,
(7.2.1)
where a{u,v) is a quadratic bilinear form (cf. Appendix A), a{u,v) : H^(Q) xH^{Q) -^ i?, and (q,v) = [ qvdx, q G L^IQ),
M = {ve H\Q); v-de
H^in); v>^,
(7.2.2)
^e
C{(1)] ,
(7.2.3)
is an example of an elliptic variational inequality, q, d and ip, are known functions. V — d on dVt is the trace sense. 0 is called an obstacle and the problem is called an obstacle problem or an elliptic variational inequality with obstacle. If there exists a function UQ E M which satisfies (7.2.1) for ah v G M, then WQ is called a solution of the elliptic variational inequality. It can be proved that A1 is a non-empty closed convex set (cf. § 7.2.4 for the proof). Problems with more than one obstacle can also be studied
150
Elliptic and Parabolic Variational Inequalities
(cf. [178]) but such problems will not be discussed here. It is well known in the calculus of variations that the problem formulated in a proposed class of functions may not possess a solution in that class. This difficulty can be overcome by broadening the class of functions in which the problem is formulated and therefore the admissible functions are considered in 'Sobolev spaces' or the 'space of distributions'. A 'Hilbert space' H^{Q) has been considered in (7.2.1) as we would like that at least the first order weak derivatives of functions belonging to M exist. The choice of an appropriate Sobolev space depends on the physical problem under consideration. For example, if in a given physical problem, a solution of the form y = \x\, x e R, makes sense then H^{R) is an appropriate space for the admissible functions. Several other questions concerning variational formulations arise which will be explained after an adequate mathematical exposition of the concepts and notions of variational inequalities. Let A : H^{Q) -^ H~^{Q) ( H~^ is the dual space of the Hilbert space i / \ i.e., the space of continuous hnear real valued functions on H^{Q)) be an operator such that for a fixed lA G H^{Q), Au{v) = a{u,v), v G H^{n). A^ defines the mapping!' -^ a(u^v). It can be easily proved that Au is linear and if a{u,v) is continuous, i.e., |a(w,i;)| < M||tx||||f || for some M e R then \\Au\\H-Hn)=
sup \Au{v)\ < M\\u\\ IHIH=I
(7.2.4) ^ ^
and hence Mll£(//i,//-i) ^ ^ 1 (constant).
(7.2.5)
Therefore, A is continuous and belongs to H~^{Q) as A is a linear continuous real valued function on H^{Q.). By Riesz representation theorem^ we have H-HnMu,v)HHn) = {u\v),
V i; e H\n),
(7.2.6)
where u* is some fixed element of H^{Q,) and depends on AuConversely, if A is hnear and satisfies (7.2.4), then a{u,v) is a continuous bilinear form. It is clear that a{u,v) = //-i(o)(^n,'?^)//i(Q)-
(7.2.7)
With each q € I/^(n), we can associate a continuous linear functional Bg G H~^{Q) which is defined as Bq:v^{q,v)=
f qvdx, v G H\n).
(7.2.8)
For a rigorous proof of this statement see [22]. A sketchy proof can be given using the following arguments. With each q G L'^i^), an element of the dual space of L'^{Q) can be associated. If the dual space of L'^{Q) is denoted by L^*(Q) then L^*(Q) •—> H~^{i'}) . In view of these arguments, we can write LHn){q.v)H^in) = H-^{Q){Bg,v)H^^n) = J qvdx = {q,v), v G H\n).
(7.2.9)
7.2. Elliptic Variational Inequality
151
The pairing {q,v) is the value of the functional Bg at v in the sense of (7.2.9). We shall not justify at other places the use of {q^v) in the place of (q^v)). The above discussion suggests that a true variational inequality (or equality) in the context of (7.2.1) should be of the form a{u,v-u)
= H-Hn){^u^^-u)H^^Q)
> H-^n)
{q^y~u)H^n)^
yveH\n),
ueH\Q).
(7.2.10)
The space //^, k > I can also be considered and weaker or stronger conditions can also be imposed on the functions involved in the variational inequality formulation. The form of a(u, v) depends on the elliptic operator considered in a given physical problem. For example, if the elhptic operator A : H^{Q) -^ H~^{Q) is defined as Au = V\
ueH\Q),
(7.2.11)
then aiu.v) = ,f Vu- Vvdx. a{u,v) Jn
(7.2.12)
If the elliptic operator A is defined as ^2^
^
Au=-Y,a^J{x)^ ~y
,
n
^^
+ Yl ^ ^ ( ^ ) ^ - + e(x)w, X G 0,
OXiOXj
~l
(7.2.13)
OXi
then the bilinear form in (7.2.1) is defined by the relation
^x^ dxj
^
\
^
dxj / dxi
The relationship between an elliptic operator and the bilinear form which occurs in the inequality will be discussed later (cf. (7.2.24) and (7.2.37)). The coefficients a^j, 6^ and e in the elliptic operator in (7.2.13) should belong to appropriate function spaces so that the variational inequality formulation makes sense. We shall see later that for the existence and uniqueness of solutions, it will be required that a{u,v) satisfies some conditions such as continuity, coercivity and these conditions also put some restrictions on the coefficients in the elliptic operator. The non-homogeneous Dirichlet problem corresponding to the elliptic operator A in (7.2.13) can be stated as the problem of finding a function u{x), x ^Q. such that Au = q, mQ]
u = d, on dVi.
(7.2.15)
If continuous derivatives of u exist in Q and d is continuous, then (7.2.15) is satisfied in the classical sense or pointwise sense. If q is square integrable in H, then Au = q is satisfied in the sense of distributions. If only the weak derivatives of u exist, then Au = q is satisfied in the distributional sense and u = d is satisfied on d^ in the trace sense.
152
Elliptic and Parabolic Variational Inequalities
7.2.2
Minimization of a functional
A minimization problem and its equivalent elliptic variational inequality The variational inequality (7.2.1) can be expressed in some other forms also. We shall first show that the variational inequality problem (z, v-z)-]-
a{z, V- z)>{q,v-
z), \/ v E M\
z E M\
q e L^{n),
(7.2.16)
where A^* is a convex set of an inner product space and Q is as in (7.2.1), is equivalent to the problem of minimization of a functional P{v), where P{v) = {v, v) + a{v, v) - 2{q, v), v G M*, q G L^Q).
(7.2.17)
Let a{u,v) be symmetric and 2: be a solution of (7.2.16). By definition a{u,v) is linear in both the arguments but it need not be symmetric. We shall show that P(z) < P{v), \/ v e M* so that z is the minimum of P{v). P(v) - P{z) = {v, v) - (z, z) + a{v, v) - a{z, z) - 2{q, v) + 2{q, z) + 2(z, V ~ z)^ a(v, v) - a{z, z) - 2{q, v - z).
= {z-v,z-v)
(7.2.18)
Also a{u, u) = a{u — w,u — w) — a{uj, w) + 2a{u, w).
(7.2.19)
Take u = z and w = z — v in (7.2.19). It can be seen that a{v, v) - a{z, z) > 2a{z, v - z).
(7.2.20)
On using (7.2.20) in (7.2.18), we get P{v) - P{z) >{z-v,z-v)
+ 2(2, V- z)^ 2a{z, v - z) - 2(q, v - z).
(7.2.21)
If (7.2.16) holds, then from (7.2.21), P(v) > P(z), V t; G M*. To prove the converse, suppose that P{z) < P{v), V z; G A^*. Since A4* is a convex set, if V G M* and z G M* then (1 - a)z + av G M*, 0 < a < 1. Therefore, P{z + a{v - z)) - P(z) > 0 , or a(v ~z,v -z) + aa(v -z,v
-z) +2{{z,v 'z)-^a{z/u
-z)-{q,v
For (7.2.22) to hold for an arbitrarily small a, (7.2.16) should hold.
-z)} >0. (7.2.22)
7.2. Elliptic Variational Inequality
7.2.3
153
The complementarity problem
By considering the minimization of the functional (a real valued function) f{v) = a{v,v)-{q,v),
(7.2.23)
we shall now obtain other forms of elliptic variational inequalities. Although a{u, v) given in (7.2.14) can also be considered, a simple bilinear form given by a{u, v)=
fVu'Vv
dx,
(7.2.24)
Q
will be considered for illustration. It will be assumed that a minimizing function u{x) exists such that f{u) < f{v), \fveM,
ueM,
(7.2.25)
where the set M is defined by (7.2.3). Since M is a, convex set, for 0 < a < 1, we have f{u 4- a{v - u)) - f{u) > 0, V ^ e X ,
(7.2.26)
or / {V(ti + a{v — u)) ' V{u + a{v — u)) — Vu • Vu}dx — n 2p{u + a{v - u)) + 2p{u) > 0.
(7.2.27)
Here, p is a continuous linear functional on H^{n) and is defined by
p{v) = I qvdx.
(7.2.28)
On using the hnearity of both p and the gradient operator, from (7.2.27) we obtain a^j V{v - u) • V[v - u)dx + aj {Vu • V{v - u) - q{v - u)}dx > 0.
(7.2.29)
For (7.2.29) to hold for an arbitrarily small a, we should have [{Vu • V{v -u)n
q{v - u)}dx > 0.
(7.2.30)
If it is assumed that u G H'^(Cl) Pi C{Q), the first term in (7.2.30) can be integrated by parts (application of Green's formula). Since v satisfies the same boundary conditions as u, we have v — u — 0 on dQ. On using this condition on d^ and doing the integration by parts, we obtain [{Vhi + q){v - u)dx <{), W veM,
u^M.
(7.2.31)
154
Elliptic and Parabolic Variational Inequalities
Let V = u + C, C > 0, C e C^{^)- liu>jp, then v > ip. Substituting ( = {v - u) in (7.2.31) and remembering that C > 0 and ( G C ^ ( 0 ) , we conclude that V'^u + ^ < 0, a.e. in Q, u> ip.
(7.2.32)
Next, consider a subset Qi of 0, where ni = {xe ft; u{x) > ^{x)} , Qi C Q.
(7.2.33)
If u{x) and ip{x) both are continuous in Q, then for every x G Qi there exists a neighbourhood of X in which u{x) — ip{x) > 0. Therefore Qi cannot be a closed set. For any C G Co°(ni), V = u — ^C is in TM provided | ^ | is sufficiently small. On substituting V = u — e( m (7.2 31), we get + q)C dx < 0.
-e f{V\
(7.2.34)
Q
If (7.2.34) holds for £ positive or negative and C is arbitrary, then it can be easily concluded that V\
+ q = 0, a.e. in ^ i , u>2p.
(7.2.35)
The set Qi in which u{x) > ip{x) is called a non-coincidence set and the set ^2= {x £ ft : u{x) = ip{x)} is called a coincidence set. The boundary of the non-coincidence set is called a free boundary. If F is the free boundary, then r^dftiH n.
(7.2.36)
On combining the results obtained in (7.2.32) and (7.2.35), we get the following variational inequality problem. Find u e H'^{Q) H C(0) such that V^u + g < 0, > a.e. in Q,
(7.2.37)
{V^u + q){u - ![;) = 0, J u-de
H^{ft).
The boundary condition u = d is satisfied on dft in the trace sense. The free boundary problem to be studied in Qi is to find a pair (w, F) such that V \ + ^ - 0 , in^i,
(7.2.38)
u =ir, 1 ^ = 1 ^ , l
OXi
on F, J
(7.2.39)
7.2. Elliptic Variational Inequality
155
u=^d on aoi n on.
(7.2.40)
The second condition on F in (7.2.39) arises due to the fact that u — ip takes its minimum on r (assuming u and ip to be smooth). The problem stated in (7.2.37) is called a complementarity problem and it is a standard problem in 'quadratic programming'. Its 'finite-difference discretization' will have the following form. BU-\-D<0, (7.2.41)
where B, [/, D and ^ are appropriate matrices obtained after discretization of (7.2.37). For example, B could be a n x n matrix and [/, D and ^ could be n x 1 matrices (n stands for the number of nodal points). On introducing the following substitutions [/ - ^ - ^ and y = -{BU -h D),
(7.2.42)
in (7.2.41), we have a problem of finding E such that BE = ~Y - {B^ -h D), E'^Y = 0; E > 0, Y>0.
(7.2.43)
Matrices B, D and ^ are known. The problem (7.2.43) is equivalent to the following programming problem provided B is symmetric and positive definite [179]. Minimize {B^ -h DfE
+ l^'^BE,
for E>0.
(7.2.44)
If a more general form of a{u,v) is considered such as the one considered in (7.2.14), then by following the procedure indicated in (7.2.26)—(7.2.35), one can easily obtain the complementarity problem of the form (7.2.37) in which in the place of S/^u we shall have Au given in (7.2.13). The variational inequality (7.2.37) can be transformed into an inequality of the form (7.2.1). Let C G C ^ ( n ) , C > 0 and li be the solution of (7.2.37). Uv = u + C, then v > ip and V belongs to M. Multiplying the first equation in (7.2.37) by C and integrating over 0, we get f{S/^u + q)C<0. n
(7.2.45)
Integrating (7.2.45) by parts and remembering that C ^ " ^ — '^ = 0 o n dQ, we get an inequality of the form (7.2.1). The arguments used in obtaining (7.2.45) hold good even if V'^u is replaced by Au given in (7.2.13). The inequality (7.2.1) also implies the problem in (7.2.37). To show this, we first consider the case u > ip. Take C ^ C'^(^), C > 0- If V = u-\- (, then we have v > ip and therefore v belongs to M. Substituting ( in place of {v — u) in (7.2.1) and integrating by parts it can be easily seen that V^n + g' < 0, a.e., in Vt provided u G H'^{Q) 0 C{^). Next consider the set ih = {xeVt: u{x) > ^(x)} C M. Follow the procedure which was used to arrive at (7.2.35). It is then easy to show that {V\ + q){u -ip) = 0 ior u>ip.
156
Elliptic and Parabolic Variational Inequalities
7.2.4
Some existence and uniqueness results concerning elliptic inequalities
The minimization problem associated with the functional in (7.2.23) and the complementarity problem in (7.2.37) are also called variational inequality problems. We shall see later that variational inequalities can be expressed in some other forms as well. The equivalence of some different formulations of variational inequalities will be discussed in § 7.2.5 in the context of Problem 1.1.12. Questions pertaining to the existence, uniqueness, and stability of the solutions of variational inequalities arise at this point. As mentioned earlier, sophisticated functional analytic tools are required to answer them (cf. [178], [180], [181]). To understand the basic concepts we shall first discuss some results on the existence and uniqueness of the solutions of elliptic variational inequalities. These results will be helpful in parabolic variational inequalities also. It may be noted that for the existence of the minimum of a function (functional) it is not necessary for the function to be continuous. For example if f{x) = \x\, x ^ 0 and /(O) = —2 then / has a minimum value —2. In this case f{x) is not continuous but is lower semi continuous (l.s.c). The conditions under which the unique minimum of some of the functionals exist are discussed below in the form of theorems which provide answers to some problems. Problem 7.2.1. Given a real vector space X, a function f : X -^ R and a set Y C X. Find the minimum of / in y, i.e., find yo ^Y such that f{yo) = infy^y /(?/)• The answer to this problem is contained in the following theorem. Theorem 7.2.1. If X is a reflexive Banach space, f : X -^ Ris a. convex and l.s.c. function, F 7^ 0 is a closed convex subset of X, and either, Y is bounded or / is coercive, then Problem 7.2.1. has a solution. This solution is unique if / is strictly convex. This theorem is called 'theorem of minimization of convex functionals'. Problem 7.2.2. Let H^ be a Hilbert space and V C W he a non-empty closed convex set, g € W {W is the dual space of W and the elements of W are linear, continuous real valued functions) and f : W ^^ R is defined by fiv)-\\\v\\lv-giv)-
(7.2.46)
Find UQ eV such that f{uo) = inf^^-p f{v). Theorem 7.2.2.
The solution to Problem 7.2.2. exists and is unique.
Proof. We shall show that / is both strictly convex and coercive and so Theorem 7.2.1. can be applied. Since VK is a Hilbert space, it is reflexive. Let A] and A2 be any two scalars such that Ai, A2 > 0, Ai + A2 = 1 and Vi and V2 {vi ^ V2) be any two vectors belonging to W. We have
7.2. Elliptic
Variational
Inequality
157
Ai \\vi\f + (1 - A,) Wv^f - {A? \\vif = Ai(l - A,) {\\vif
+ 2Ai(l - A0(vi,«2) + (1 - Ai)^ \\v2\f} - 2(vuV2) + \\v2f}
,
(7.2.47)
> A , ( l - A i ) { | K f - 2 | h | | | h | | + ||t,2f},
(7.2.48)
> A i ( l - A i ) { | | t ) i | | - | | w 2 | | } ^ > 0 , if wi 7^ W2 and Ai ^ 0.
(7.2.49)
Also /(Ai?;i + A2V2)
=
I IIAit^i + A 2 i ' 2 | | ^ - s ( A i f i + A2i'2)
[A? I b i f + A2 ||t)2f + 2X,\2(vuV2)] - Ai5(«i) - A2ff(t;2) 2\{\A\vi\?
<
<
+ ^2\\v2f] - \l9{vx) - \2g{v2)
)^lfivi)+\2f{v2)-
Therefore / is strictly convex. Prom the Riesz representation theorem, we have
giy)<\\9\\JHw.
(7.2.50)
On using (7.2.50) in (7.2.46), we get
fiv) > ^ll<-NL,IHk, >
7 ||z;||
, for some positive constant 7 as [It'll -^ 00, V v,
(7.2.51)
and so / is coercive. P r o b l e m 7 . 2 . 3 . Given a Hilbert space W, g e W and P 7^ 0, a closed convex set of W, find UQ E V such t h a t (wo, UQ — v)w < g{uo — v), ^ V eV.
(7.2.52)
T h e o r e m 7.2.3. ?io is the solution of Problem 7.2.2. if and only if UQ is a solution of Problem 7.2.3. or in other words Problems 7.2.2. and 7.2.3. are equivalent. P r o o f (in outhnes). First, we prove t h a t Problem 7.2.2. implies Problem 7.2.3. If UQ e V and V E V, then UQ + a{v — UQ) e V for 0 < a < 1 diS V is a convex set. If the minimum is attained at UQ, then F(a) - f{uo + a{v - UQ)) > / ( ^ o ) , and F ' ( 0 ) > 0. Let G{a) = UQ -\- (y{v - UQ), then
(7.2.53)
158
Elliptic and Parabolic Variational
Inequalities
T h e first term on the r.h.s. of (7.2.54) can be easily calculated from the first principles and we obtain ^ f ^ ilG(a)ll^)
={uo,v-uo)w
(7.2.55)
On using the hnearity of g
i ^(«(-))
= -J- (giuo) + ag{v - uo))
= g{v - Uo)
(7.2.56)
On combining the results in (7.2.55) and (7.2.56), we obtain (7.2.52). To prove t h a t Problem 7.2.3. implies Problem 7.2.2., it will be assumed that the solution of Problem 7.2.3. exists. For the existence proof, see [22]. It can be easily shown t h a t if the solution exists then it is unique. Let ui and U2 be two solutions of Problem 7.2.3.. We have (til, ui - U2)w < 9{ui - U2), U2 e V,
(7.2.57)
and {u2,U2-ui)w
< 9{u2-ui),
ui eV.
(7.2.58)
Adding (7.2.57) and (7.2.58) and using the definition of 'scalar product' and the linearity of ^, we get {ui - U2, ui - U2) < 0,
(7.2.59)
which implies Ui = U2. In view of the fact t h a t Problem 7.2.2. implies Problem 7.2.3. the unique solution of the Problem 7.2.3. is also the unique solution of Problem 7.2.2. T h e inequality (7.2.52) is also called a variational inequality and here the inequality is defined with the help of a scalar product, whereas, in (7.2.1) it has been defined with the help of a bilinear form. T h e functional (7.2.46) is a special case of the functional f{v) = ^a{v,v)-g(v),
(7.2.60)
where a : W x W -^ R is a, bihnear form. If with each pair {u, v) € W x W, a(u, v) associates the scalar product {u,v)w or in other words a{u,v) — (u,v)w then |a(ii, i;)| = |(ti, t')H/| < TII^IIWII'^IIW^ (7 = 1) and a is continuous. a{u,v) = {u,v)w = {v,u)w — a(v, u) and a is symmetric, a has also the coercivity property on W, i.e., a{u, u) > a | | n | | ^ for a = 1 as {u,v) = ||tx|p > ||ti|p. On the other hand if a is coercive, symmetric and continuous then a scalar product ((u, v)) can be defined with the help of a bilinear form
{{u,v)) = a{u,v),
V u, V e W.
(7.2.61)
Let III. Illv,/ be the norm associated with the inner product defined in (7.2.61). We shall show t h a t \\.\\w and |||. |||v^ are equivalent and therefore the continuous linear functionals
7.2. Elliptic Variational Inequality
159
defined on W for the two norms are the same. ce||u||^ < a(u,u) = {{u,u))w =|||'w|||u^= 7 ll^lliy • Therefore the two norms are equivalent and a{u^v) is coercive and continuous with respect to the norm ||H||M/. Equivalence of norms implies that if a{u, v) in (7.2.60) is symmetric, coercive and continuous with the respect to ||.||v^, then the functional / in (7.2.46) is no more general then the functional in (7.2.60) as the functional in (7.2.60) can be written as f{v) = l\\H\w-9iv),
(7.2.62)
where |||I'|||VK = (('^5'^)) = ^l'^^'^), v e W. Next we ask whether the minimum of the functional (7.2.60) can be obtained as the solution of a variational inequality. Problem 7.2.4. Let 14^ be a Hilbert space, V C W, a. non-empty closed convex set and f : W —> R a, functional defined by (7.2.60) in which the bilinear form a{u, v) is continuous. Find UQ E V such that / ( ^ o ) < / ( ^ ) , ^veV. Problem 7.2.5. UQ eV such that
(7.2.63)
Let W, V, f and a{u,v) be the same as in Problem 7.2.4.. Find
a{uo, uo-v)<
g{uo -v),
^ veV.
(7.2.64)
If UQ is the minimum of f{v) in (7.2.60), then -^/(i/o + (y{v - iio))|a=o > 0, V 1; G p . (7.2.65) da The derivative in (7.2.65) can be easily calculated by using the bilinearity of a and the linearity of ^ and it can be shown that (7.2.63) imphes (7.2.64) only if a(w, v) is symmetric. If a solution of Problem 7.2.5. exists, then it can be proved that it is unique [22]. If it can be proved that a unique solution of Problem 7.2.4. exists, then the equivalence of Problems 7.2.5. and 7.2.4. can be established. If a is coercive onV — V, i.e., 3 Of, Of > 0, e.g., a{u — v,u — v) > a\\u — v\\l^ , \/ u,v eV,
(7.2.66)
then the solution of Problem 7.2.5. exists [22]. V - V = {x - y\ x e V, y e V}. It can be proved that if a is symmetric and, continuous on V and coercive oxiV — V then it is coercive on V. a{u, u) = a{u- (,u> a\\u-(\\l >a\\u-
()-
a(C, C) + 2a(ii, (), u^V,
-7||C|PvK -27lMII|Clk, O
0, 7 > 0
Cfw -7llClk (3||Clk +2 11^ - Clk).
It is now easy to prove that a (w, n) ^- +00 as ||i/||^- 00.
( fixed in V
(7.2.67)
160
Elliptic and Parabolic Variational Lions - Stampacchia
Inequalities
Theorem
T h e o r e m 7.2.4. Let W he a Hilbert space. V C W he a non-empty closed convex set, g E W and a : W x W ^ R, a continuous bilinear form on W and coercive on V — V. There is one and only one UQ ^ V such that a{uo, v-uo)>
g{v - UQ), \/ veV,
(7.2.68)
and further the application which associates UQ to every g is continuous, i.e., the Problem 7.2.5. is well-posed. Let V = M (for M see (7.2.3)) in Theorem 7.2.4. so that we can discuss stability of the solution of the variational inequality (7.2.1). A comparison of (7.2.68) and (7.2.1) suggests t h a t the linear functional g is defined as g{v) = {q, v)=
[ qvdx, ^ v e M. n
(7.2.69)
Note t h a t a test function v satisfies the boundary condition. Therefore, to prove continuous dependence of the solution on the data, it is to be proved that if ui and U2 are two different solutions of (7.2.68) corresponding to the d a t a qi and q2, then a\\ui-U2\\^<
\\qi - q2\\w'inr
« > 0-
C^-^-^O)
Let gi and p2 be associated with qi and q2 through (7.2.69). From (7.2.68), we have a(ui,U2 — ui) > gi{u2 — Ui), {take v = U2)
(7.2.71)
a{u2,ui — U2) > g2{ui — U2), {tdkev
(7.2.72)
and = ui).
Adding (7.2.71) and (7.2.72) and changing the sign, we obtain a{ui -U2,ui
-U2)
< g\[ui - U2) - g2{ui ~ U2) < \\9i - 92\\w'\\ui - U2\\w-
Using the coercivity of a{u,v) tt|K
on P , (7.2.73) can be written as
- U2\\w{n)< Iki - q2\\w'in)^ ^ > 0-
This proves continuous dependence of the solution on the d a t a . The Problem generalization of Problem 7.2.3. as it imposes weaker conditions on the form There is an interesting geometrical interpretation of the minimization of the (7.2.46). Using Riesz representation theorem, f(v) in (7.2.46) can be written fiv)
(7.2.73)
= IMIV
- («*. " ) , V e P,
u* e W.
(7.2.74) 7.2.5. is a of a{u/u). functional as (7.2.75)
7.2. Elliptic Variational Inequality
161
Define a functional F{v) as
F{v) = f{v)-^l\y\\l = \\\yfw-{u\v)w
+ \\\u*\\l
= \\\v-urw
(7.2.76)
If there is an element of V which minimizes F, then it also minimizes / and minimizing F amounts to finding 8i v E V whose distance from a fixed u* E W is minimum, i.e., ||tiQ — tx*|| is minimum. U UQ eV is such that ||iio — i^*|| is minimum then IK - u*\\<\\v - n*||, \fveV.
(7.2.77)
It can be proved that the inequality (7.2.77) is equivalent to the following inequality {uo-u*,v-uo)>0,
yve
V.
(7.2.78)
Hint: let UQ, U* and v be the vertices of a triangle. Use the proposition that the sum of the two sides of the triangle is greater than the third side. Problems 7.2.2. and 7.2.3. are both equivalent to the problem of finding UQ E V for a given u* (determined by g) satisfying (7.2.78) for \/v EV. UQ is called the projection of u* on V. The inequality (7.2.78) is yet another way of writing the variational inequality associated with functional (7.2.46). Some of the results discussed above can be proved under weaker conditions by considering a reflexive Banach space in the place of a Hilbert space but in a Hilbert space more interesting results can be obtained which in some cases have interesting interpretations. In (7.2.64) (we consider this inequahty as it is more general than (7.2.46)), g is hnear. Therefore, (7.2.64) can be written as a(no, uo-v)-
g{uo) < -g{v),
^ veV.
(7.2.79)
This suggests that we can consider variational inequalities of the type a(u, v-u)+
p{u) < p{v), y veV,
(7.2.80)
where p : W -^ Ris not necessarily linear. In view of (7.2.80), we want to study now the minimization problem associated with the functional f{v) = m{v) + p(v)] f :W -^ RU {oo},
(7.2.81)
where p is not hnear and m{v) could be a functional whose Gateaux derivative denoted by \/m{v) exists. The motivation for considering Gateaux derivative comes from the fact that in seeking the minimum of a function defined from R -^ R we look for those points at which the classical derivative of the function is zero. On using the linearity of
162
Elliptic and Parabolic Variational Inequalities
a{u, v) it was possible to calculate the derivative in (7.2.65) and therefore the Gateaux derivative of the bihnear form a{u, v) exists. If the continuity of a{u, v) is assumed, then it can be proved that the mapping v -^ a(u,v) is continuous (see (7.2.4)). What type of functional m{v) should be? Note that p{v) can be easily handled as it is enough if it is l.s.c. and a proper convex functional. A question similar to the one raised about the minimum of the functional in Problem 7.2.4 can be asked for the functional (7.2.81). Problem 7.2.6. Given a Hilbert space W, V CW, a, non-empty closed convex set and f : W -^ RU {-j-oo}, a functional of the form f{u) = m{u) -\-p{u)\ find UQ E V such that f{uo)
yveV.
(7.2.82)
The conditions under which UQ is the solution of (7.2.82) are given in the following theorem. Theorem 7.2.5. If m{u) is finite, convex and G-differentiable {Gateaux differentiable) on V and p is convex and a proper functional on V, then UQ E V is the minimum of f{u) in Problem 7.2.6. if and only if UQ satisfies the inequality ^ , (Vm(iio), ^^0 - v)^
+ p{uo) < p{v), VveV.
(7.2.83)
The inequality (7.2.83) presents yet another form of variational inequality. We shall not pursue further the generalizations of the functional whose minimum could be obtained. Variational equation li V = W in Problem 7.2.4. and we seek the minimum of / for V t; G W, then in (7.2.65), a could be any real number and not restricted to 0 < a < 1. In this situation, in the place of (7.2.65), we shall have an equation f'{0) = 0 and we get a variational equation to determine the solution no and we have a{uo,z) =g{z), zeW.
(7.2.84)
However, this is not the case always. For example, if / is of the form (7.2.81) in which m{v) is G-differentiable but not p{v), then even ii V = W, we have an inequality of the form (7.2.83) (cf. [22]). The bilinear form a(ii, v) in (7.2.14) is not symmetric and therefore the scalar product which by definition is symmetric cannot be defined in terms of a(w, v). The bilinear form (7.2.24) is symmetric and let us examine what happens if a scalar product is formally defined in terms of a(u, v) by the equation (u,v) = a{u,v)=
js/u-Vvdx, n
u,v e H\n).
(7.2.85)
All other properties of the scalar product are satisfied by (u, v) if it is defined by (7.2.85), except the property {u,u) = 0 => u = 0. We have already seen that if a bilinear form
7.2. Elliptic Variational Inequality
163
can be equated with a scalar product then the biUnear form is continuous and coercive and both these properties are very useful in analysing the solutions of the variational inequalities. It is possible to define a scalar product with the help of (7.2.85) after some modifications in the formulation of the variational inequality. Let Q C i?'^ be an open bounded set with dQ of class C°, i.e., every point of dQ has a neighborhood U such that dQnU IS the graph of a continuous function (cf. [182]), u G //^(O), k e N and V^u (a-th order weak derivative, see Appendix D) G 1/^(^2), |a| < k. H^ can be provided with the scalar product {U,V)HH^)
=
E
(^V^"^)L^(^),
(7.2.86)
and (^,^)//Mi^)= E f\D''u\'dn. N
(7.2.87)
The norm ||ti||//fc(m = (u.uY^'^. The scalar product in HQ{Q) {HQ{Q.) is the closure of the space C^{Q) in VK^'^(Q), see Appendix D) can be defined as {^.y)H^in) = E {D''u,D-v)mn).
(7.2.88)
\a\=k
In the space HQ{Q), the equivalence of the norms generated by the scalar products defined in (7.2.86) and (7.2.88) can be asserted with the help of the following theorem. Theorem 7.2.6. Let Q C /?" be an open bounded set and /c E TV (N is the set of positive integers). There exist two constants /Si and /32 such that for every u G HQ{D.) the following inequality holds.
J2 f\D^u\^dx\
<
(E/P"^I'^^)
< /^2( E
[iD'^^l'dx]
.
(7.2.89)
The inequality on the l.h.s. of (7.2.89) can be easily proved and we have /?i = 1. To prove the inequality on the r.h.s., the Poincare's inequality [22] can be invoked which states that if ^ G C^Q) then
Jw'dx
av^''
^X^
dx,
(7.2.90)
for some constant /? > 0. Since HQ{Q) is the closure of the space C^(l]), (7.2.90) holds even if t/; G HQ{Q). If the r.h.s. in (7.2.90) tends to zero, then the l.h.s. also tends to zero which implies ip = 0. The above discussion suggests that the scalar product can be
164
Elliptic and Parabolic Variational Inequalities
defined with the help of (7.2.88) provided the space HQ{Q) is considered in the place of
H\n). Next we show that the existence and uniqueness results are not affected if the space HQ{Q) is considered in the place of H^{Q.). Let u be the solution to the variational inequality f Vu ' V{v - u)dx > f q{v- u)dx, \/ v e M, u e M, n n
(7.2.91)
where M is defined by (7.2.3). U w = u — d, then w = 0 on dQ and w e HQ{Q). U is the solution to (7.2.91) if and only \i w — u — d\s the solution to / S/w ' V{v - u)dx >{q,v-u)-jvd-
V{v - u)dx.
(7.2.92)
We have, v — u = v~d — w = C,—w, and both C and w belong to Hl{Q) C H^{Q). The inequahty (7.2.92) can be written as fVwV{C-w)dx> (FX-w), VCeMi, n
(7.2.93)
(F, C) = {q, C) - / Vd • VCdx, 4 , e L^(Q), 1 < z < n,
(7.2.94)
where
and Mi = {Ce H^{n) :C>^IJ-d}.
(7.2.95)
It can be seen that if Oi{i^ - d) + {1 - a)(V^ - d) > ip - d, for 0 < a < 1. Therefore A^i is a convex set. The functions of HQ{Q) are absolutely continuous functions if fl is an open subset of R^. Let ip be continuous on Q = (0,6). Consider the function [ip—d]'^ = [ip—d+\ip~d\]/2. [tp—d]'^ is the positive part oiip—d, therefore [(l)-d\^ > ip—d. AsC = '^ — t/ = Oon dQ, 'ip — d < Q on dQ and so [ip — d]'^ = 0 on dQ. Mi is non-empty as it contains [ip — d]"^. We shall now show that the space Mi is closed (complete in the norm defined through (7.2.88)). Let {vn} be a convergent sequence in Mi whose limit is V. It is to be proved that v e Mi- The sequence {vn} can be thought of as a subsequence of a sequence {vn} in HQ{Q) and {vn} converges to v G HQ{Q) in the a.e. sense. Since Vn G Ml, Vn'>^ for V n. {v^] is a subsequence of [vn] therefore v > ip and so 1) G M\. In the space HQ{Q), scalar product can be defined as {w, C) = a{w, 0= [ VwVC dx.
(7.2.96)
7.2.
Elliptic
Variational
Inequality
165
We have a(w,w) = {w,w) >\\w\\l|2 and therefore a{w,() is coercive and continuous on HQ{Q) or coercive on A^i — A l i - T h e conditions of Theorem 7.2.4. are satisfied and a unique solution WQ of (7.2.92) exists in Aii. The unique solution UQ of (7.2.91) is then given by UQ = WQ -{- d. Till now only Dirichlet problem has been considered for the elliptic operator of the form V^. We now consider a problem of the form -V\
+ Xu = q, a.e. in Q, u e H^(n)
n C(Q), q G L^{n),
(7.2.97)
u = d, a.e. on dQ, d e L^ {9) .
(7.2.98)
The differential equation (7.2.97) can be obtained as an Euler equation of an appropriate minimization problem, for example, in (7.2.60) take a[u^v) given by (7.2.100), g defined by (7.2.69) and v G H'^{Q) D C{Q). If an obstacle is introduced such a^s v > ip then the obstacle problem for (7.2.97) —(7.2.98) can be formulated as a variational inequality of the form (see the derivation of the complementarity problem in (7.2.37) obtained from the minimization problem) given below in (7.2.99). a{w, C-uj)>
{qX-uj),\f
Ce H^{Q),
w G //d(^),
(7.2.99)
\f w, C ^ ^ o ( ^ ) -
(7.2.100)
where a(wX)=
fVw.VCdx n
+ xfwCdx, n
It can be proved t h a t if A > —1//?, where P is the same as in the Poincare's inequality (7.2.90) then a{wX) is coercive on H^{Q.) (cf. [22]). If the boundary condition is of the Neumann type such as - ^ = r, a.e. on dQ, r G L^(Q),
(7.2.101)
where ft is the unit outward normal to dVt, then the obstacle problem results in a variational inequality of the form a{u,v-u)
> f q{v-u)dx+
f r{v - u)dx,
V i; G H\n),
ue
H\n).
(7.2.102)
In this case a{u, v) has the same form as in (7.2.100) but u,v e H\n) (boundary condition is different). By choosing A > 0 and z/ = m i n ( l , A ) , it can be shown that (cf.
[22]) a{v,v)>u\\v\\l,^,,^,
(7.2.103)
where the norm in //^(Q) is defined by (7.2.86). Note that the Neumann boundary condition (7.2.101) has already been incorporated in (7.2.102). The test function need
166
Elliptic and Parabolic Variational Inequalities
not satisfy the Neumann boundary condition as it occurs naturally in the inequality. When — V^w -^ Xu < q is multiplied by {v — u) and integrated over Q and integration by parts is done we get (7.2.102). The integration by parts will give an integral over dQ, and the Neumann boundary condition is incorporated. If A = 0, the form a{u, v) is not coercive, and in this case the problem with Neumann condition does not always have a solution; and when it does, it is not unique. The boundary conditions could be of mixed type, for example, let dQ = dQi U aOs, a^i 0 ^ ^ 2 = 0 and - V ^ u + A = ^, a.e. inQ,
(7.2.104)
u = d, a.e. on 5^1, (7.2.105) Ou — = r, a.e. on aOa. (7.2.106) on Then the variational inequality formulation is still given by (7.2.102) with dfl in the integral on the r.h.s. of (7.2.101) replaced by ^^2- Since a{u,v) remains unchanged, in this case also, (7.2.103) holds good and a{u,v) is coercive. The test functions have to satisfy the boundary condition (7.2.105). Operators such as the one considered in (7.2.13) or even more general operators can be considered and the existence and uniqueness of solutions of the variational inequalities so obtained can be discussed under suitable assumptions (cf. [22]). It is not always possible to formulate a given physical problem as a variational inequality problem or obtain a variational equation for the problem. Regularity of the solution is an important aspect of the study of variational inequalities. Some results on the regularity of solutions will be given in the context of parabolic variational inequalities. With the help of the obstacle problem of the string discussed earlier in Problem 1.1.12., some other aspects of variational inequalities such as the choice of appropriate spaces in which the solutions are sought, and the restrictions on the obstacle so that the set A^ in (7.2.3) is non-empty, will be discussed in the next section. In general the space of unknown functions should be large enough so that the existence of solutions can be discussed, but small enough so that a unique solution can be obtained. In physical problems, smoothness of solutions cannot be ignored. In principle, the data spaces should be general so as to accommodate various types of data but the continuous dependence of the solution on the data is required for well-posedness.
7.2.5
Equivalence of different inequality formulations of an obstacle problem of the string
We restate briefly an obstacle problem of the string as a problem of finding a continuous function v(x) which minimizes the energy functional f{v), where f(v) = ^J^{v'fdx,
v€M2,
(7.2.107)
7.2. Elliptic Variational Inequality
M2
=
{ve
=
{veH^{0,b):
167
H\0,b):v{0)
= v{b) = 0,
v{x)>i;{x),y
V{X)>IIJ{X),\/xe{0,b)}
xe{0,b)}
(7.2.108)
Since we are dealing with a physical problem, v and tp should be continuous functions. It is clear from Fig. 7.2.1. that the first order derivatives of v{x) and ip{x) need not be continuous but they should belong to L'^{ft) which is also suggested by the integral in (7.2.107).
Fig. 7.2.1. Differentiability of i;(a:) is not required.
Fig. 7.2.2. The set M2 is empty.
In Fig. 7.2.2., although v{x) > il){x)^ v does not satisfy the boundary conditions and so the set AI2 is empty. For M2 to be non-empty, 1/; must satisfy one of the following two conditions. (V^l) : 7/; G C^(n), t/;(0-|-) < 0 and i/;(6-) < 0.
(7.2.109)
(V^2) : il) e H\n),
(7.2.110)
V^(0) < 0 and t/;(6) < 0.
Note that by increasing the smoothness of -0, the smoothness of the solution is not increased. For example, if tp is taken as a parabola then ip G C°°(Q), fl = (0,6), but still the second derivative of the solution u" ^ C^(0, 6) if the parabola does not pass through the points 0 and b. In showing the equivalence of different forms of variational inequality formulations given below for the obstacle problem, the treatment is not strictly rigorous. A rigorous treatment would require that the condition u" < 0 is considered in the sense of distributions in the formulation (I) given below. If w G i/^(0,6), then it is not necessary that u" belongs to L^(0,6). It will be assumed for the present discussion that u,v e //^(n)nC(Q) and u" belongs to L^(Q). The obstacle ip satisfies one of the two conditions (-01) or ('02). For clarity, the three different formulations are restated below. (I) Find ue M2 {M2 as in (7.2.108)) such that u{0) {u - 'ip)u" = 0.
u{b) = 0, w'' < 0, 16 > 0 and
168
Elliptic and Parabolic Variational Inequalities
(II) Find u such that b
I u\x){v'{x)
- u'{x))dx > 0, ^ ve M2, u e M2.
(7.2.111)
0
(III) Minimize j{dvldxfdx,
\J
v^M2.
If u',v' e L^n), then u'^ and u'v' G L^Q). Further let u" G L\Q). assumed in the following that all the equations hold good in the a.e. sense.
It will be
Equivalence of formulations (I), (II) and (III) (III) ^ (II).
Assume that there exists a w G M2 such that b
b
fu'^dx<
fw''^dx,
0
^ w eM2.
(7.2.112)
0
Since M2 is a convex set, ii u,v G M2 then w = u + X{v — u) e M2, 0 < A < 1. From (7.2.112), we get b
b
ju'^dx
< l{u'^ + X\v
- u'f + 2\u'{v' - u')}dx,
0 < 2A I u'{v' - u')dx + 0 ( A 2 ) .
(7.2.113)
(7.2.114)
0
For (7.2.114) to be true for V A > 0, we should have 6
Iu'{v' -u)dx
> 0.
0
(II) => (III).
If a(w, v) is defined by (7.2.24), then a{u - v,u-v) b
b
J2u\u'
fu^dx< 0
-v')dx-
ju'^dx+
fv'^dx-2fu\v'-u)dx< 0
0
>0 implies
b
fv'^dx>0,
fv'^dx.
(7.2.115)
(7.2.116)
b
As (7.2.111) holds, the relation ./Q ?/(i/ - u')dx > 0 can be used in (7.2.116). The inequality (7.2.116) implies that u gives the minimum in (III).
7.2. Elliptic Variational Inequality
169
(I) ^ (11). b
0
/ u{v' — u)dx
= u{v — U)Q — / u"{v — u)dx,
0
0 6
=
- j u{v-u)dx.
(7.2.117)
0
Let 0 = (0,6) = /i U /2, where Ii = {x eQ: u{x) = ^p{x)}; u" < 0 in A if formulation (I) holds,
(7.2.118)
l2 = {x en-.
(7.2.119)
u{x) > ip{x)}; u" - 0 in /2 if formulation (I) holds.
Using (7.2.118) and (7.2.119) in (7.2.117), we get h
fu{v'
- u')dx = - j u"{v - ^{x))dx > 0.
0
(7.2.120)
h
The integration by parts in (7.2.117) requires that u" should belong to L^{Q). Therefore (I) ^ (II) provided the solution u of (I) is such that u^' G L^{Q). FVom Fig. 7.2.1 it is clear that the solution may exist even if u" ^ L^{Q). It has been shown in [183] that if ip be such that ip" is a Radon measure and its positive part can be represented by a function belonging to L^(0, 6) then the condition u" ^ L^{Q) is not required. (II) => (I). Consider (7.2.117) and the case u>ip. and V e M2- If (II) holds, then
Letv = u+C. C > 0, C ^ C'^(^),
h
- fu"(dx>0.
(7.2.121)
0
The relation (7.2.21) implies u" < 0 in fl. Next consider the case u> if). Let v = u-\- eC,, ( E CQ^{I2)- Extend C to Q trivially, i.e., C = 0 in / i . If £ is sufficiently small, v will be greater then ijj and we have h
h
I u'{v' - u) dx = - j u"{v -u) 0
0
dx= -£ j u"C dx > 0. /
(7.2.122)
2
Since (7.2.122) holds for all sufficiently small £ which could be positive or negative, it can be concluded that fu'Xdx h
= 0.
(7.2.123)
As C is arbitrary, u" = 0 on /2. The equivalence of different variational inequality formulations of problems involving more general elliptic operators can also be discussed. For example, it can be proved that the variational inequality (7.2.1) with a{u,v) defined by (7.2.14) is equivalent to the variational inequality (7.2.37) in which the operator A is given by (7.2.13) provided ?i, the coefficients in the elliptic operator, and the obstacle ip satisfy some conditions [22].
170
7.3 7.3.1
Elliptic and Parabolic Variational Inequalities
The Parabolic Variational Inequality Formulation in appropriate spaces
Let Q be an open bounded domain in K^ and Qt^ = 0 x {0 < ^ < ^*}. One of the problems of mathematical physics is to solve the initial-boundary value problem stated below in (7.3.1)-(7.3.3). Tt + AT T T =
= /, i n ^ ^
(7.3.1)
= g, ondQt.,
(7.3.2)
To, in n X {0} ,
(7.3.3)
where A = -f: au{x. t ) ^ ? ^ + E H ^ ^ t)^^^^ iT^i oxidxj ^ dxi dQu =dnx
+ d{x,t)T{x,0,
{0,t,).
(7.3.4)
(7.3.5)
dQt^ U 17 X {0} is called a parabolic boundary. The weak form of problem (7.3.1)—(7.3.4) can be stated in terms of finding a function T{x,t) € H^(QtJ satisfying the equation ( T „ v - T ) + a{t;T,v-T)
= (f^v-T),
(7.3.6)
for a.a. t G (0, t^) and all v G 7/^(0^ J . v satisfies the given initial and boundary conditions,
a(,T,.) = / ( E a . . £ | ; ; + E^.|;- + '^^^k ^^ = ^^ + E?^^
(7.3.7) (7-3.8)
ox J
= Ivwdx.
(7.3.9)
It has been assumed in (7.3.4) that the data, the temperature derivatives with respect to x and i, and coefficients a^j are suflBciently smooth functions (cf. [1] and proposition 7.3.1.). There does not exist a minimization principle for parabolic problems. Often Biot's variational statement [184] is taken as a variational principle. Every elliptic problem cannot be generalized to a transient problem, for example, the obstacle problem for the string (Problem 1.1.12.) does not have a time dependent generalization. If an obstacle is introduced in the formulation (7.3.1)—(7.3.4), then we may obtain an inequality.
7.3. The Parabolic Variational Inequality
171
Parabolic variational inequality for the problem in (7.3.1)—(7.3.4) can be defined in any of the following two equivalent forms. (I)
For a.a. t e (0,^=,), let E{t) =^{ve H\n);
V > i){x,t) a.e. in Q].
(7.3.10)
E{t) is a closed convex subset of H^{Q). To make E{t) a non-empty set, we shall take ilj[x,t) < g{x,t) a.e. on dQ for t G (0,^*). A function T{x,t) G L^{0,t^] H\n)),T{t) e E{t), for a.a. t G (0,^*), i.e, for each fixed t, T{x,t) = T{x) G H^{Q) is called a solution of the parabolic variational inequality (7.3.11) if {Tt,v-T)-^a{t]T,v-T)>{f,v-T),
for a.ate{0,t,),
(7.3.11)
is satisfied for V ^; G L'^{0,t^;H^{n)) with v{t) G E{t) for a.a. t G (0,^*), v{x,0) = To and V satisfying (7.3.2). It is required that dT/dt G 1/^(0, i*; L'^{n)). ip{x,t) is called an obstacle, /(x,^), ip{x^t) and dQ should satisfy some smoothness conditions. Some of these conditions will be discussed for some specific problems to be discussed later. (II) The second variational inequality formulation is the integrated form of (7.3.11), i.e., in the place of (7.3.11), we have r^* dT Jo { ( ^ ' ^ - ^ ) + ^ ( ^ ' ^ ' ^ - ^ ) - ( / ' ^ - ^ ) } ^ ^ > 0 -
C^-3-12)
Let E = {ve L'^{0,t^]H\n));
v>ip{x,t)
a.e. in Q^., v{x,0)=To}.
(7.3.13)
v{x, t) satisfies the boundary condition (7.3.2). If there exists a function T(x, t) E E with dT/dt G 1/2(0, t^] L'^i^)) such that (7.3.12) is satisfied for all v e E, then T{x, t) is called a solution of the variational inequality (7.3.12). Any solution of (7.3.11) will satisfy (7.3.12) and so the formulation (I) implies formulation (II). We assume that a solution T{x,t) of (7.3.12) exists. For some £ > 0 such that (^0 ~ ^5 ^0 + ^) ^ (0, t^), consider the following function. V
=
T[x,t),
t i
{to-£,to-\-£
=
V,
t e {to - £,to-\-£)
(7.3.14) If V defined in (7.3.14) is used in (7.3.12), then to+e
'71 dT J U^.v-T) ^ dt
+ a{t;T,v-T)~(f,v-T)\dt
> 0.
(7.3.15)
to-e
Since the interval (^o — £,to + £) G (0,^*) is arbitrary, we obtain (7.3.11). The complementarity problem corresponding to the parabolic variational inequality (7.3.11) in which
172
Elliptic and Parabolic Variational Inequalities
the operator A is given by (7.3.4) consists of finding T{x, t) satisfying the following system of equations. Tt + AT>
/,
T>ip{x,t),
\ inQt^.
(7.3.16)
{Tt + AT-f){T-,l;{x,t)) = 0,. The relations in (7.3.16) are satisfied in the a.e. sense and T G L^ (0,^*;//^(O)). The equivalence of the variational inequahty (7.3.12) and the complementarity problem (7.3.16) can be estabhshed. The proof is similar to the one discussed in the elliptic case (see § 7.2.3). Consider first the case T > ip, where T is the solution of (7.3.12). If 0 G C^(QtJ, 0 > 0 and 0 (x, 0) = 0 then v = T-{-(j) belongs to the set E.
/'
(Tt + AT -f)(t>dxdt = fli-^^y-
fit,
T)+a{t] T, v - T)-{f,v-
T)\dt > 0. (7.3.17)
0 '^
Since 0 is arbitrary, we conclude that dT/dt -\- AT — f > 0 in Q^,. Next, we consider the case T > ip and use the procedure followed in the elliptic case (equations (7.2.32)—(7.2.36)) with a difference that now v = T — scf), \£\ > 0, £ very small, 0 € C^i^u) and (p{x,0) = 0. It can be seen that dT/dt + AT - f = 0. Combining the two cases T > ip and T > ip, (7.3.16) is obtained. To prove that (7.3.16) implies (7.3.12), consider the first integral on the l.h.s. of (7.3.17). On replacing 0 by (z; — T), writing v — T = v — ip— (T — tp) and using the third relation is (7.3.16) and the fact f > -0, we obtain {Tt + AT-f){v/
T) dxdt > 0, \f v e E.
(7.3.18)
•
fit»
Since (7.3.18) holds for all v e E,T is the solution of (7.3.12). In the elliptic case, the coercivity of the bilinear form played an important role in proving well-posedness and uniqueness of the solution. If the transformation f=
e-"*T, a > 0 ,
(7.3.19)
is used then Tt + AT = f is transformed into f;-h (/I + a) t = e - " 7 , and a{t]T,v)
(7.3.20)
is transformed into d{t] t , v) = a{t] f,v)-^a
( t , v).
(7.3.21)
If a is sufficiently large, then irrespective of the fact that a(t; T, v) is coercive or not a{t] T, v) is coercive. Without any loss of generality it can be assumed from the start that a(t]T,v) is coercive and if for a.a. t, T G H^{Q) then a(^;T,T) > A||T||^i > A / ( | V T | V T 2 ) c / a : , n
A>0.
(7.3.22)
7.3. The Parabolic Variational Inequality
173
Uniqueness and stability of the solution of (7.3.12) can be proved if coercivity condition is imposed on a{t]T,v) in (7.3.12). Let Ti and T2 be two different solutions of (7.3.12) satisfying the same initial and boundary conditions. Taking v = T2 in (7.3.12) and T = Ti, we obtain
'm^ T2 - Ti
+ a{t; T,T2 -T,)
- [f^T^-TM
dt> 0.
(7.3.23)
Similarly by taking T = T2 and v = Ti, a. second relation is obtained. Adding the two relations so obtained, it is easy to obtain the relation / U ^ (Ti - T2, Ti - T2) + a{t;T, - T2,T, - T^) \dt<0.
(7.3.24)
On integrating the first term under the integral in (7.3.24), remembering that Ti — T2 = 0 at t = 0 and using the coercivity condition (7.3.22), we obtain. t*
\ \\Ti - T^ilL. + A / lITi - r2||^,(„) dt < 0.
(7.3.25)
0
Since both the terms in (7.3.25) are positive, each should be zero and this implies Ti = T2. The stability of the solution can be proved by following the proof given earlier in the elliptic case (cf. § 7.2.3 equation (7.2.74)). For proving the existence of the solution penalty method is generally used. The conditions under which a unique solution of the variational inequality (7.3.11) exists for the parabolic operator given in (7.3.4) have been discussed in [1] and are briefly given below in Proposition 7.3.1. Proposition 7.3.1. Assume that the following conditions are satisfied. (1) The operator Tt + AT, where A is given by (7.3.4) with coefficients defined in Vtt, is of parabolic type at {x,t), i.e., n
E
«ij(^. *)^'0 > A lel' , V ^ G i?", A > 0.
(7.3.26)
It is said to be uniformly parabolic in Vtt^ if (7.3.26) holds for all (x, t) in Q.t^ and A is independent of (x,t). (2)
E li«.^lla + E ll'-.L + NIL < M(Constant), 0 < a < 1,
(7.3.27)
where \\T{x,t)\l
= \\T{xM,
||T||o=
+ H.{T{x,t)),
sup|T(x,i)|,
(7.3.28) (7.3.29)
174
Elliptic and Parabolic Variational Inequalities
HM=
sup
f^--^)-^^^^^^^^^^^^^
Pdiix, 0, {x\ t')) = \x- x'\ + 1^ - t'\"^.
(7.3.30) (7.3.31)
Pd{{x,t), {x',t')) is called parabolic distance. (3) dVt is in C2+^ (cf. Appendix D). (4) d{x,t) > 0 and /, g, D^g, Dig, Dtg belong to CiClt^). C"(OtJ is the space of functions u{x,t) which are Holder continuous with exponent a, Ha{u) < oo. (5) \Dxaij\ < Ml (constant) so that a{t;u,v) can be defined.
(7.3.32)
If the above conditions are satisfied, then there exists a unique solution of the obstacle problem (7.3.11) ((7.3.12)) and D^T, DlT, DtT belongs to ^^(f^f J , Vp, 1 < p < oo.
7.4
7.4.1
(7.3.33)
Some Variational Inequality Formulations of Classical Stefan Problems One-phase Stefan problems
The variational inequality formulation is a fixed domain formulation and therefore weak enthalpy formulations of the Stefan problems are more suitable for variational inequahty formulations than the classical formulations of Stefan problems. Even in the classical Stefan problems, variational inequality formulations of one-phase Stefan problems can be handled easily than the two-phase Stefan problems. This is because a one-phase Stefan problem can be formulated in terms of the 'freezing index' which together with its gradient is continuous throughout the region. Also the constraint that the temperature is greater than or equal to (less than or equal to) the melting temperature holds throughout the phase-change region. We describe below some variational inequality formulations of one-phase Stefan problems. Some results concerning analysis of solutions of these problems are also given. Consider an open bounded domain Q in R^. At time t = 0 , 0 is filled with ice cold water at temperature T = 0 where 0 is the dimensionless freezing temperature of water. Let dQ = dQf{t) U dQ,e{t) U drtr{t) and on these three disjoint portions of the boundary, three diff"erent types of boundary conditions are prescribed. On the portion dQj coohng is done so that ice formation takes place. For ^ > 0, ^^ = ^o{t) UQi{t) U r(^). rto{t) is the ice region at time t, 0 < t < t^, ^i{t) contains water at temperature T = 0 and r(^) is the
7.4. Some Variational Inequality Formulations of the Classical Stefan Problems
175
ice-water phase-change boundary whose equation is taken as f = (p{x)^ x ^ Q. Qo{t) and ^i{t) are disjoint regions. The mathematical formulation of this problem is as follows: ^
= V. {kVT),
in QQW, 0
T = 0, i n ^ i W ,
0
T(x, t) = g{t) < 0, on ^Q/, 0 < t < t^,
(7.4.1) (7.4.2) (7.4.3)
T(x, t) = 0, on dQr, 0
(7.4.4)
dT aT = A:--, on dQe. 0
(7.4.5)
[x) = Ic ; and T (x, 0 (a:)) = 0,
k\/T'V(t)
T{x,0) = 0 i n ^ .
(7.4.6) (7.4.7)
Here, n is the unit outward normal on dO^e^ Ic = l/C > 0, /c is the thermal diffusivity and a is the constant for heat radiation. It has been assumed that r{t) is monotone and ice is progressing on water. Although the constraint T < 0 holds throughout ^, the variational inequality formulation of this problem presents two difficulties. The above formulation is not a continuum model (because of the free boundary conditions) and secondly VT is discontinuous across t = (t){x) and so integration by parts which is required to obtain the bilinear form cannot be done in Q unless we write the integral as the union of two integrals (see equation (7.3.7)). DuvaiVs transformation Using DuvaiVs transformation [185], temperature T(a:,t) is transformed into another dependent variable u(x,t), called freezing index [186], such that u and Vu are continuous throughout Q. u[x,t) is defined as t
u(x,t) = I T{x,T)dT, in Qo(0; ^ ( ^ . 0 = 0 in Oi. Note that u{x,t) is continuous in Q x (0, t*) . From (7.4.8), we can easily obtain the equation t
Vu{x, t)=
f VT(x, T)dT - T(x, 0(x)).V0(2:), in QQ
(7.4.8)
Elliptic and Parabolic Variational Inequalities
176
Vu{x,t)=
I VT{x,T)dT,
(7.4.9)
inQo(^),
{x)
Vu{x,t) = 0, mVLi{t). Also
(7.4.10)
t
V • (kVu)
=
/ V • (kVT)dT - kV{T{x, (/)(x))).V0(2:) 4>{x)
} ^^,
1
0(x)
[ Tdr - I,,
di
Hx)
du
du
'di -V '{k\/u)-lc If u - 0, then du/dt - V. (^V^; du ~di
(7.4.11)
/c, in Qo(0.
Jt
(7.4.12)
= 0, inQo(0-
: = -lc< 0. Therefore
V • {k\/u) -lc<0,
inn,
0
(7.4.13)
< t*.
The initial and boundary conditions for u(x,t) can be obtained from those given for T(x,t) and the problem for the dependent variable u(x,t) can be stated as follows: du/dt - V • {kVu) - /c < 0, in Qt. = ^ x (0, ^*), ] u<0,
inQt.,
\
u(du/dt - V • (kVu) - /,) = 0, in Qt..
(7.4.14)
J
t
u{x,t) = [ g{T)dT = g{t) < 0, and ^(0) = 0; on dQfit), 0
(7.4.15)
0
u{x, t) = 0, on d^rit), auix,t) = k^,
(}
on dQeit), 0
(7.4.16) (7.4.17)
on u{x,0) = 0, inQ. The above equations are to be understood in the a.e. sense.
(7.4.18)
7.4. Some Variational
Inequality
Formulations
of the Classical Stefan Problems
177
A variational inequality of the form (7.3.12) will now be obtained for equations (7.4.14)-(7.4.18). Define a set D as
where V < 0 a.e. in Qt., v{x,0) i; = 0, on dQr, 0 V = g{t), on d^f,
= 0 in Q,
(7.4.19)
The test function v{x^ t) need not satisfy (7.4.17) as it will appear as a natural boundary condition in the formulation. It can be seen t h a t {{ut, V -u)
= {ut-V
- {kVu) -Ic,
+ {kVu, V{v - u)) - (/c, V -
V- u)^ + {kVu -n, V- u)^^^,
u)}^
0 < ^ < ^*.
(7.4.20)
The last term on the r.h.s. of (7.4.20) arises on doing the integration by parts of the term {kVu, V ( f — u)). On dVtr and dVt, v — u = 0. In view of the third condition in (7.4.14), we have {ut - V. (kVu)
-lc,v-
u)^ = {ut - V. (/cVu) - /c, v)^ > 0.
It may be noted t h a t in Qt., Ut — V • (kVu) (7.4.17) in (7.4.20), it can be written as
— Ic < 0 and v < 0. On using (7.4.21) and
{{ut, V — u) + {kVu, V{v — u)) — {Ic, V — U)}Q — {au, v — for a.a. te
(7.4.21)
(0,^*).
U)QQ^
> 0, (7.4.22)
In (7.4.22), the boundary conditions (7.4.17) and (7.4.16) have been incorporated. Neumann boundary conditions are called natural boundary conditions because they are automatically taken care in the variational formulation and the test functions are not required to satisfy them. Dirichlet boundary conditions are to be satisfied by the test functions. On integrating (7.4.22) with respect to time, we get t*
I [{{uu v-u)-\-
{kVu, V {v - u)) - (/c, V - u)}^ - {an, v - U)Q^^ dt > 0. (7.4.23)
0
The inequality (7.4.23) is the parabolic variational inequality associated with the one phase Stefan problem (7.4.1)—(7.4.7). If there exists a function u £ D {D as above)
178
Elliptic and Parabolic Variational Inequalities
such that (7.4.23) is satisfied for all v £ D, then u is called a solution of the variational inequality (7.4.23). Hue L'^{0,t^;H\Q)) and the norm of u is defined as ^ -"^ J Il"ll//i(f2)
(7.4.24)
0
then L^(0, ^*; H^{Q)) is a Banach space. The formulation given in (7.4.14)—(7.4.18) is also a variational inequality formulation and in this case we are looking for u e L'^ (0, t^; H'^{Q)). It has been proved in [187, 188] that if the meas {dQf) > 0, then there exists a unique solution u(x,t) of the variational inequality (7.4.23) subject to the initial and boundary conditions (7.4.15)—(7.4.18) such that u e L\0, t,] H\n))
n L°°(0, t,; L^{n))] SLudu-V'
(kVu) G ^ ^ ( a j .
(7.4.25)
The numerical solution of the variational inequality {u,v —u)Q-\-{kVu,V{v —u))Q-\-{au,v — u)dn^'> {lc,v — u)n,
for a. a. ^G(0,^*),(7.4.26)
has been discussed in [188] by 'Galerkin approximation' in space and discretization by finite-diff"erence in time. A variational formulation of a two-phase Stefan problem has been given in [189] and its numerical solution has been obtained by finite-difference methods. The initial condition (7.4.18) imphes that at t = 0, 0 is occupied by ice cold water. We shall now consider the problem in which at t = 0 ice occupies a region Co with temperature g{x) < 0, Go C ft. ^ — GQ is filled with water at temperature T = 0. Let Qo{t) be the region occupied by ice at any time t and Qi{t) be the water region so that Q = Qo{t)UQi{t)[jr{t), where, QQ and ^ i are disjoint regions and r(^) is the ice-water interface whose equation is given by t = (p{x). li x e Go, then 0(a;) = 0. The initial and boundary conditions are given by (7.4.16)—(7.4.18). We assume that ice is progressing on water. The freezing index u(x,t) in this case is defined as u{x, t)=
f T(x, T)dT, in no(t) -Go,
0
t,,
(7.4.27)
4>{x)
t
u{x,t) = fT{x,T)dT,
in Go, 0
^*,
(7.4.28)
0
u{x,t) = 0, in Qi(0, 0 < ^ < ^*; and u{x,0) = 0, in Qi(0).
(7.4.29)
The above transformation was suggested in [190]. It is easy to check (see (7.4.11)) that V • {kVu) = ut- Ic, in ^o{t) -Go,
0
t,, k = IjG,
(7.4.30)
7.4. Some Variational Inequality Formulations of the Classical Stefan Problems V • {kVu) = ut- g[x), in Go; and 7i = 0, in Qi(t), ^
179 (7.4.31)
Define a function f{x) such that / ( x ) - p ( a ; ) , ^ ( x ) < 0 , rrGGo(x), ] i f{x) = lc>0, xen-Go. J
(7.4.32)
In view of (7.4.30)-(7.4.32), we have ut - V.{kVu) - / = 0, w < 0, in Qoit), 0
(7.4.33)
and when ti = 0, we have u = 0; W ' f - V. (/cVti) - f = -f = - / „ x € Q - r2o(0, 0 < ^ < ^,.
(7.4.34)
Combining (7.4.33) and (7.4.34), we get u {ut - V. (k\Iu) - / ) = 0, in Q, 0 < ^ < t^.
(7.4.35)
For (x,t) € r^f,, (ut - V. (/cViz) - / ) (t; - ti) -
-(?it-V.(A;Vw)-/)?i+(7it-V. (Ww)-/)^;,
> 0, a.e. in ^u, v< 0, in ^t.-
(7.4.36)
Therefore, the variational inequahty problem corresponding to (7.4.30)—(7.4.32) can be stated as the problem of finding u G 0 such that [uu v-u)^^r
{kVu, V{v-
u))^ -{f.v-
u)^ - {au, v - u)^^^ > 0,
for a.a. t G (0,t*) and \/ v e D.
(7.4.37)
Here, D is the same as defined in the context of problem (7.4.14)—(7.4.18). By taking Q = {a: : 0 < x < i?o < oo} , and Q = ^0(0 UQi(^) u r ( t ) , where Q.Q(t) and Q.\{t) are disjoint regions and r(^) is the phase-change boundary, a one-dimensional onephase problem has been considered in [178]. Oo(0) = {x : 0 < x < ^o, 0 < 5*0 < Ro) is the region initially occupied by hot water at the temperature h{x) > 0. r2i(0) = {x : So < X < RQ} is occupied by ice at the melting temperature zero. A variational inequality with u = g{t) > 0 (sufficiently smooth) prescribed at x = 0 has been studied in [178] when water is progressing on ice, i.e., w > 0 in Q{t^) = 0(0) x (0,^*). Using the 'penalty method' (cf. [1]), it has been shown that under suitable assumptions there exists a unique solution u to the variational inequality such that u, w^, Ut, i/xx e L'^iQit,)),
(7.4.38)
and u > 0, Wf > 0 in Qo{t^). The region Qo{t) = {x : u{x, t) > 0} , 0 < ^ < ^* is expanding continuously with time. Further, the water-ice boundary F admits the representation F : x = a(t),
0
180
Elliptic and Parabolic Variational Inequalities
where cr is a continuously increasing function of t with ^o = a (0) < a (t) for ^ > 0. For each (xo, ^o) ^ T, there exists a neighbourhood Br of (XQ, ^O) such that Uxx: Uxt ^
C{QonBr).
The one-phase Stefan problem has been studied by many authors with the help of the variational inequality formulation and references of several such studies can be found in [191]. In the water region, convection can be included in the formulation (cf. equation (7.4.43) given below). It is possible to obtain the complementarity problem in a constantvelocity case by applying Baiocchi transformation [192] t
u{x, t)=
f T{x, T)dr, xeQx
(0, t^),
(7.4.39)
0
to equation (7.4.43). Even in this simple situation, it is not possible to convert the complementarity problem to a variational inequality formulation due to difficulties arising in the transformation of boundary conditions and the geometry of the domain (cf. [191]). Several problems connected with the one-phase Stefan problem, such as, the exterior problem, the continuous casting model, and the degenerate case pertaining to a quasisteady state model have been studied in [191]. In the interior problem, the melting of ice takes place due to a prescribed non-negative temperature on one part of the boundary of Q C R^ and the Neumann boundary condition prescribed on the remaining part of the boundary of Q. Some results of the regularity of the freezing index and its continuous dependence on the data in a strong sense can be found in [191]. The geometry of the exterior problem is similar to that of the expanding core model in the Hele-Shaw problem (cf. § 6.2.1). The variational inequality formulation of the exterior problem can be obtained in terms of the freezing index by using transformation (7.4.39). As discussed earlier, for large times, cusp-like singularities may develop on the free boundary. A strong geometric assumption about the data in the exterior problem leads to a star-shaped configuration without singular points [190]. A fundamental question which arises concerning the solution of a variational inequality is its relationship with the classical solution of the Stefan problem in which the temperature of the phase-change boundary is the melting temperature and the energy balance condition is satisfied pointwise on it. This question was essentially solved in [193, 194]. For the smoothness of the free boundary and the 'Caff"arelh's criterion' on the local existence of the classical solutions, see references [193] and [195]. Under some assumptions it has been proved that for each ^ > 0, the n-dimensional Lebesgue measure of the free boundary t = (t){x) is equal to zero; (j) [x) is a locally Lipschitz function and there exists a neighbourhood Go of the point (x,t) on the free boundary where the free boundary can be represented in suitable coordinates by the graph of a C^ function in the form Xn=^S{xuX2,
Xn_i,^), SeC\
and all the second derivatives of u are continuous upto the free boundary.
(7.4.40)
7.4. Some Variational Inequality Formulations of the Classical Stefan Problems
181
One-phase continuous casting model and its variational inequality formulation The formulation of the problem and the analysis presented below is not classical. This problem is being discussed here to introduce a continuous casting model and because of some novelty in the expressions of bilinear form and the inner product used in the formulation. The thermal energy conservation equation in a heat conducting body ft C R^ is given by de p-- + V-q = 0, mnt.=^x{0,t,), at
(7.4.41)
where e is the specific energy and ^is the heat flux vector. If the phase-change is taking place in Q at temperature T^ and the latent heat / is released, then e can be expressed as e = CT + lH,{T-Tm).
(7.4.42)
where Hy{T) is the Heavyside function {Hy{x) = 0, if x < 0 and Hy{x) = 1, hi x < 1). If the body is moving with a velocity v, then the total derivative with respect to time is to be taken and (7.4.41) can be written as ^
+ ^- V r - V.(VT) = -l(~
+ ^- v V ( T ) ,
(7.4.43)
For simplicity, it has been assumed in (7.4.43) that C = 1.0, p = 1.0, A; = 1.0, T^ = 0 and / is a constant. The equation (7.4.43) holds in Qt^ in the distributional sense and for its derivation see [196]. We consider ice-water system in which the temperature of water is zero and ice is progressing on water. If 9 represents the difference between the solidification temperature and the actual temperature of ice, then (7.4.43) can be written as (cf. [196])
do ~+v-Ve-
fd V^e =-ll-+v.\/\
\ X(,>o), a.e. in Q,.,
(7.4.44)
where x is the characteristic function of the set {^ > 0}(x = 1 for {9 > 0}) which is now ice region and water region is the set {^ < 0}(x = 0 for {^ < 0}) . If the ice-water sharp boundary is written as t = (p{x), x e R^, x = (xi, X2, x^), then the classical Stefan condition and the isotherm condition can be written as {V^ • V(/) -h / (1 + ^ • V(/))} (T, 0 (x)) = 0; and 9 = 0.
(7.4.45)
To complete the formulation, the initial and boundary conditions should be added to (7.4.44) and (7.4.45). We shall now discuss variational inequality formulation of a problem arising in connection with the formation of the continuous ingot by the process of continuous casting.
Elliptic and Parabolic Variational Inequalities
182
A simple diagram depicting solidification of the melt in continuous casting is shown in Fig. 7.3.1. As soon as a sufficiently stable core is formed, the platform B begins to drop down with the velocity v in the direction xi and thus it draws out a continuous cast of cooling liquid. For appropriate assumptions under which a simple mathematical model of continuous cast is formulated, see [21]. The portion of the ingot taken into account in the present formulation occupies a cylindrical open bounded domain Q = Qi x Q2 C R^, n = 2, 3 where ^1 = (0, 6), b is the height of the lateral mold and ^2 == (0, a) if n = 2 because of the symmetry, or if n = 3, r^2 C i?^ is a domain with Lipschitz boundary 8^2 representing half the section of the ingot. Let FQ = {0} x 1^2, Ti = (0, 6) x ^^2, and r2 == {i>} X ^2- A point X e Cl has the coordinates x = {xi,x') where x' = X2 if n = 2 and x' - (X2, X3) if n = 3. In the notation V = (^i, V ) , V = 82 ii n = 2 and V = (^2, ^3) if n = 3. Here, ^i, 82 and ^3 denote the partial derivatives with respect to Xi, X2 and X3, respectively. The initial temperature of the melt is given by
e{xux\o) = Oo{xux')
(7.4.46)
x,=s(x„t)
_ Mold surface beingcooled
Solid J—Platform B
Fig. 7.4.1. Formation of continuous ingot Boundary conditions which are prescribed on the lateral, bottom and upper surfaces of the cylinder are : 6'(0, x\ t) = 0, on Fo, t > 0, -Ve'n
= a{e-
Oi) on Fi, 6>i > 0, a > 0,
e{h,x',t) = e2{x\t),
onF2, ^ > 0 .
(7.4.47) (7.4.48) (7.4.49)
The geometry considered in [191] is different from the geometry shown in Fig. 7.4.1. as only one branch of the free boundary has been considered in [191]. By symmetry consideration, one branch of the free boundary can be considered in Fig. 7.4.1. also but the boundary condition at 2:2 = 0 will be different from that at X2 == a and not as in (7.4.48).
7.4. Some Variational Inequality Formulations of the Classical Stefan Problems
183
The boundary Fi is being cooled, n is the unit outward normal on Fi. The energy equation in Qt* = O x (0, t*) is given by ^^~d^~^^^^~\m^^^'d^)^^^''°^' ^'^' ^^ ^**•
(7-4.50)
Here, v = {VQ, 0,0) and VQ is the constant casting velocity in the direction of xi. Equation (7.4.50) holds in the sense of distributions. Let the free boundary be given by xi = S{x\t),
x' G ^2, t > 0,
(7.4.51)
where S{x', t) is a smooth function, S" : Q2 x (0, t^) —> [0, b). The conditions at the free boundary can be written as 0 = 0] &nd-^-V0.VS = l(vo-^). OXi
(7.4.52)
Ot
The second condition in (7.4.52) can be obtained by using relations of the type (1.4.19) and (1.4.21) and remembering that d[xi — S)/dt = VQ — dS/dt. The region Xi > S{x', t) is the solid region (see Fig. 7.4.1.) and the region Xi < S{x',t) is the melt region. Using the maximum principle, it can be proved that under the boundary conditions considered, for Xi > S{x',t), 6 > 0. Only dS/dt < VQ is physically admissible. In order to obtain variational inequality formulation of this casting problem, the following transformation has been used in [196] which regularizes the equation (7.4.50). t
u{x,t) =Vo f 6{XI-\-VO{T-t),x',T)dT,
x = {xi,x') eQ, t>Q,
(7.4.53)
0
where ^ is a non-negative function and is extended to —00 < Xi < S {x',t) by taking it to be zero in this extension. If ^ G Lj^^(QtJ, then 6 can be obtained in terms of u by the relation (cf. [196]) du du —- +Vo^— = voO, a.e. in Qt.. at oxi
(7.4.54)
It can be shown that u{x,t) formally satisfies the following equations. (JU
du
-^ + ^ 0 ^
V^u = fX{u>o)^ a.e. in Qt,, u>0,
u = 0, on Fo; and u = 0, at ^ = 0, ^ du
= a{g-u), du
onFi,
„, , ^
(7.4.55) (7.4.56) (7.4.57)
„
,
,
184
Elliptic and Parabolic Variational Inequalities
where f and g in (7.4.55) and (7.4.57) are given by (cf. [196]) /(x, t) = VQOO^XI - ht, x')Xj
g{xux\t)=
f
Oii^.t-^-
J
- Ivo [1 - X/], x ^ ^ . ^ > 0,
Vo
-,x)d^,
0<xi
t>0.
(7.4.59)
(7.4.60)
[xi-bt] +
Here, [xi — bt] is the positive part of Xi — bt and Xi is the characteristic function of the set / related to the initial position of the free boundary $(0), and / is given by I = {{xi,x\t)
e^ty,
xi > S{x\0) + bt},
(7.4.61)
/ c n+ = {{xux\t) e a.; xi > S{x',t)} = {e> 0}, $(0) -.xi = S{x',0).
(7.4.62)
The equations (7.4.54) and (7.4.55) hold in the sense of distributions and the reader is referred to [196] for their derivation which is not straightforward. For t > b/vo, X/ = 0 ^^^ X{u>o) > X/ ^^ ^ > 0 because dS/dt < VQ. TO write the variational inequality formulation, we introduce the following notations. a{u,v)=
fVu'Vv
+ vofv-^-\-afuv,
u{t), v{t) e Mi{t),
{u,v) = [uv + — [uv, u{t), v{t) e Mi{t), J n
{Git),v) = Jf{t)v
VQ
J r2
+ ajg{t)v-h
Je2{t)v, v{t) e M,{t),
(7.4.63)
(7.4.64)
(7.4.65)
For a.a. t G (0.^*) M{t) = [v e H\n)-
V = 0, on Fo} , Mi{t) = {v e M{t)- v > 0, on 0} . (7.4.66)
The problem (7.4.55)—(7.4.58) has the following variational inequahty formulation. {du/dt,v-u)
+ a{u,v-u)>
(G{t),v-u),
a.a. t e {0,t,),
\fv(t) G Ml, u{t) e Ml.
(7.4.67)
If there exists a ii{t) G Mi(^) such that (7.4.67) holds for all v{t) G M[(f), then u{t) is called a solution of the variational inequality (7.4.67). If may be noted that the boundary conditions (7.4.57) and (7.4.58) occur as natural boundary conditions in the fornnilation and so only the boundary condition on FQ has been imposed on v.
7.4. Some Variational Inequality Formulations of the Classical Stefan Problems
185
Under suitable assumptions, existence of the unique solution of the variational inequality (7.4.67) has been discussed in [191, 196]. If has been shown that the free boundary has in fact the representation (7.4.51) such that dS/dt < VQ and {9, S) is the classical solution of the continuous casting problem. Oxygen - difTusion problem In the background of the previous formulations, variational inequality formulation of ODP discussed earher in § 3.3.2 does not present much difficulty. We consider equations (3.3.34)-(3.3.38) and take Q = [0,1], Vtt^ = Q x (0,^*), Qoit) = {x; 0<x < S{t)}, 0 < t < t^. In the region Oo(t), the concentration c > 0 and in Q\0o(^), c = 0. Therefore in 0, c > 0. Note that in ODP, the region under consideration is only Qo{t) and no constraint of the form c > 0 is imposed. The variational inequality problem is studied in the fixed domain Q, and the constraint c > 0 has been added to study an obstacle problem. It can be seen that
n 0 on
- /
\^
0 an0
^ '
dc\ (^ - c) ^
' dc d dx+ I I - ^ — {v - c) dxdt + f [ {v-c)
dxdt.
(7.4.68)
0 0
The test function v{x,t) satisfies the same initial and boundary conditions as the concentration c{x, t) and so the second term on the r.h.s. of (7.4.68) is zero. On rearranging equation (7.4.68), we obtain -
+ //(.0 0
iv - c) dxdt+ jj^--{v-c)
c) dxdt = / / ( I
- £
dxdt
+ l) vdxdt.
(7AM)
We have c^ — c^x H- 1 = 0 if c > 0 and Q — c^x + 1 is equal to 1 if c = 0. Therefore, Q — Cxx + 1 > 0 for c > 0 and c {ct — Cxx + 1) = 0 in Q. As i; > 0, the last term on the r.h.s. of (7.4.69) is greater than or equal to zero and we get Jlicuv-
c) + ( ^ , -^{v-
c) J + (1,^; - c)\dt
> 0.
(7.4.70)
This completes the variational inequality formulation of ODP. If there exists a function c{x,t) e E, where, E = {v e L^(0,t*; H\Q)) ; v > 0, a.e. in Q,^,t;(x,0) = c(x,0)}, and c(x,t) satisfies (7.4.70) for all v e E then c(x,t) is called a solution of the variational inequality (7.4.70). In the multi-dimensional case when 0 C -R", n > 1, the variational inequality formulation can be obtained following the procedure adopted in (7.4.68)—(7.4.70). The integration by parts will then require the use of Green's formula.
186
Elliptic and Parabolic Variational Inequalities
If oxygen-diffusion takes place in a subset of /?", n > 1 then the initial concentration has to be suitably prescribed as in this case it is difficult to obtain it from the solution of a steady-state problem which was possible in the one-dimensional case. The existence, uniqueness and regularity of the solution of (7.4.70) was studied in [74]. Some regularity results for the one-phase Stefan problems have been reported earlier in this section and for further information, see [197, 198, 199]. Is it possible to obtain a one-phase Stefan problem from its variational inequality formulation? Consider the problem of finding a function u{x,t) satisfying the following complementarity problem. Ut — Uxx > /, "^ > i'i^^t),
and {ut — u^x — f){u — tp) = 0, a.e. in Qt^,
nt.=nx{o
n = {x:0<x
u{x,0)
= uo{x) > ip{x,0), X G n,
u{0,t)
= u{t)>^p(0,t),
0
(7.4.71)
(7.4.72)
One can study the above variational inequality problem by employing standard methods for parabolic equations involving well known a priori iZ-estimates [9]. The solution is generally smooth in the sense that the distributional derivatives Ut^ Ux, Uxx are in LP, p > 1. Ut and Uxx are not continuous in general. Now suppose that u{x, t) > 'ip(x, t) if and only if 0 < a: < S{t), 0 < t < t^, S{0) = 1. In this case, we have Ut - Uxx = f in 0 < x < S{t), u{S{t),t) = 'ip{S{t),t) (cf. § 7.2 equations (7.2.38) —(7.2.39)). If a new function T{x,t) is defined as Tix,t) = j^iu-7P),
(7.4.73)
then T(x, t) satisfies the following equations. Tt - Txx = Ft = {f-i;t
+ V^xx),, 0 < X < S{t), t > 0,
(7.4.74)
r ( x , 0) = To{x) = ^'(x) + (/ - iJt)t=o. 0 < x < 1,
(7.4.75)
T(0, t) = f{t) = ut~ ^PtUo^ t > 0,
(7.4.76)
T{S{t),t)
= 0, t > 0, 5(0) = 1,
Tx {S{t),t) = X(S{t),t) S(t) = (/ + w.. ~ 'M \x^sit)S{t). t > 0
(7.4.77) (7.4.78)
7.4. Some Variational Inequality Formulations of the Classical Stefan Problems
187
By choosing F, TQ{X), f{t) and X{S{t),t) suitably in (7.4.74)-(7.4.78), a one-phase Stefan problem or an ODP can be obtained. On inverting (7.4.73), we get t
u (x, t)=
j T{x, rj)dr] -f V^(x, t),
(7.4.79)
4>{x)
on the free boundary t - S~\x) = 0(x). From {7.4.79), u{S{t),t) to derive (7.4.78), calculate Ut — u^x from (7.4.79) at a: = S{t).
= if; {S{t),t). In order
The equivalence of the weak formulation of a one-phase Stefan problem and its variational inequality formulation has been discussed in § 11.2.2.
7.4.2
A Stefan problem with a quasi-variational inequality formulation
Our interest in this section is only in introducing the quasi-variational inequality. For a rigorous discussion of the quasi-variational inequality, see [22] and the references mentioned there in this connection. One of the forms of quasi-variational elliptic inequality considered by BensoussanGoursat-Lions (cf. [200]), and further discussed in [22], can be defined as follows: Let Hi and H2 be two Hilbert spaces and v = (iii, ^2) be the generic element oi H = HiX H2. Define a : H x H -^ R and g : H -^ R as follows: a{w, z) = ai{wu zi) -h a2{w2, Z2) + bi{wuZ2) + 62(2^2, ^^i), g{w) = gi{wi) + g2{w2).
(7.4.80) (7.4.81)
Here, ai : Hi x Hi —^ R and a2 : H2 x H2 —^ R are continuous bilinear forms, symmetric and non-negative along the diagonal, bi : Hi x H2 —^ R and b2 . H2 x Hi -^ R are continuous bihnear forms, gi : Hi -^ R and g2 • H2 —^ R are continuous linear functional. Let D C H = Hi x H2 he a non-empty closed convex set. li v e D, then define D{v) = Di{v) x D2{v) where Di{v) = {zi G Hi : {zuV2) 6 D} , and D2{v) = {22 e H2 : {vuZ2) G D} . (7.4.82) An inequality a{u,u-
v) < g{u - v), \fv G D{u), ue D
(7.4.83)
is an example of a quasi-variational inequality. D{v) is a non-empty closed convex set of D^ = PH^{D) X PH2{D), where P//^ and P//2 are 'projection operators'. \iu ^ Hi and VK C //i is a non-empty closed convex set, then Pw{y) is that element of W which is nearest to u. The inequality (7.4.83) is a variational
188
Elliptic and Parabolic Variational Inequalities
inequality when for V 2i G D, D{u) = Q = Qi x Q2 with Qi being a non-empty closed convex set of Hi and Q2 being a non-empty closed convex set oi H2. v E D{u) suggests that quasi-variational inequality entails a convex set of competing functions which may depend on the possible solution whereas in the variational inequality formulation convex set is fixed. A family of variational inequalities can be associated with the quasi-variational inequality (7.4.83). If u is fixed in D, then along u, we can consider a variational inequahty concerned with finding ai w E D(u) such that a{w, w -v)
< g{w -v),
\/ v € D{u).
(7.4.84)
A quasi-variational inequality formulation for the following one-phase Stefan problem has been studied in [201, 202]. Find a function T{x,t) and the curve T : x = S(t), 0 < t < t^ such that ^« - Txx = 0, 0<x<
S{t), 0
(7.4.85)
T = X{x,t), X = S{t), 0
(7.4.86)
T, - 0, x = S{t), 0
(7.4.87)
T{x, 0) = To(x), 0 < X < ^0, S{0) = So, T{0,t) = r{t),
0
(7.4.88) (7.4.89)
If the transformation u = —T^ (see § 3.3) is used, then the boundary condition (7.4.87) becomes u = 0 and (7.4.86) becomes w^ = —KS — Xt- The coefficient —A^; can be identified with the latent heat and A^ with the displacement of the free boundary. If appropriate conditions are imposed on the initial and boundary data, then the problem (7.4.85)—(7.4.89) can be identified with a phase-change problem. Global existence of the classical solution of the transformed system has been discussed in [203] when A^ = 1, Ai < 0, -T^ (x) > 0 and r'(^) > 0. It turns out that in this case S(t) > 0 and u(x, t) > 0 if 0 < X < S{t). Let \{x,t), To(x) and r{t) be given smooth functions for 0 < x < RQ, 0 < t < t^ and So G (0, Ro). A quasi-variational inequality formulation of problem (7.4.85)—(7.4.89) will now be considered in the region D = {{x,t), 0 < x < RQ, 0 < ^ < f*} in terms of a new variable W{x,t) defined as W{x, t)=
(T (<e, t)-X
(e, t)) d^. {x, t) G D.
(7.4.90)
JX
It is understood that if 0 < ^ < t*, then S{t) < RQ. We extend T{x,t) as X{x,t) in the region S{t) < x < RQ, 0 < t < t^ and thus W{x,t) = 0 if S{t) < x < RQ, 0 < t < t,_
7.4. Some Variational Inequality Formulations of the Classical Stefan Problems
189
Let Q = {{x,t) : 0 < X < S (t), 0 < t < t^} . From (7.4.90), dW/dt can be calculated as follows: rRo
Wt =
1^ {T,(tt)-Xt(^,t)}di,
(x,t)eD,
= £*"{%(ei)-A<(C,()}C =
n (S(«), t) - nix, t) - r " A,(^, t)di, Jx
= -T,{x,t)-
r'\t(^,t)d^.
(JAM)
Jx
For {x,t) G r^, we have W:, (x, t) = -{T (x, ^) - A (x, t)),
(7.4.92)
W^^ (x, t) = - T , (x, t) + A^ (x, t).
(7.4.93)
and
W{x,t) satisfies the differential equation -VK^, (x, t) + ly, (x, t) = /(x, t), for (x, t) e D,
(7.4.94)
where rS{t)
f{x,t)
= -A.(x,0-/ -
A , ( e , t ) C (x,^)Gl),
-A^(x,i) if (x,t) e DV^.
(7.4.95)
f{x,t) depends on S(t), i.e., on the solution. It may be noted that f{x,t) has been suitably extended to the region D\fl as it is required afterwards in the formulation. It will be assumed that A^ > 0, A^ < 0 and To(x) > A(x, 0), 0 < X < ^o; and T{So, 0) = A(6'o, 0).
(7.4.96)
We define a function g(x) as follows: g{x)
=
r
(r(^, 0) - A(C 0)) d^,iiO<x<
So,
Jx
= 0, if So < X < R,
(7.4.97)
so that W(x,0) — g{x) and g'{x) < 0. Because of the assumption that To(x) > A(x,0) in (0,5*0), the free boundary F starts at So- The boundary condition for W{xJ,) at x = 0 is given by W,{0,t) = ^(t),
(7.4.98)
190
Elliptic and Parabolic Variational Inequalities
where ^{t) = -r{t) + X{0,t),
0
Assume that "^{t) < 0 for 0 < t < t^. It can be shown that W is greater than zero in Q. On differentiating (7.4.94) with respect to x, we obtain -W,..
+ W:,t = /x = - A . . + A„ in a
(7.4.99)
It will be assumed that -Kx + Xt < 0 in i^. Since W^, = X {x, t)-T
(7.4.100)
(x, ^) = 0 for (x, t) e T, and ^(^) < 0, we obtain -I^xxx + VP^xt < 0, in n,
(7.4.101)
W:, < 0, on dpQ,
(7.4.102)
and
where dpQ is the parabohc boundary of Q. Assuming W^ to be continuous in Q, the maximum principle (see Appendix C) implies that H4 < 0 in Q and on integrating Wx, it can be concluded that W{x,t) > W(S{t),t) = 0. The variational inequality formulation of the problem (7.4.85)—(7.4.89) can now be obtained in terms of W{x, t) as follows: Find W{x,t) e M = {v{x,t) € H\D), (-M4x + Wt) {v-W)>
v > 0} such that
f{v - W), a.e. in L> and V z; G M, W e M, W{x,0)
= g{x), 0<x<
PI4(0,0 = W{Ro,t)
(7.4.103)
Ro,
^(^), 0 < ^ < ^ * ,
= 0,
0
For 14^ > 0 and i; > 0, we have
{-Wx.^Wt~f){v-W)
= -{-W,.
+ Wt-f)W
= {-W,x +
+ {-W,, +
Wt-f)v,
Wt-f)v.
=
0, if H^ > 0 and = -fv
>
0, for V I) G M, W e M.
if I^ = 0, (7.4.104)
provided / is negative in D\^, which it is, under the assumption that A^^ > 0 (see (7.4.95)). It is clear from (7.4.95) that / is a function of {x,t] S{t)) and S{t) =
7.4. Some Variational Inequality Formulations of the Classical Stefan Problems
191
min {x, W{x, t) = 0} . Therefore / is a discontinuous function of W{x, t) and (7.4.104) is not a variational inequality in the usual sense. In [201], authors call it a quasi-variational inequality. The existence of the solution of (7.4.103) has been proved in [201] by considering a sequence of variational inequality problems in which the approximations of the free boundary are taken as known. Fixed point theorem for monotone mappings was used in [201] for the existence proof and in [202] existence of the solution of the above problem has been proved using finite-difference approximations and this resulted in a smoother solution. Under appropriate smoothness assumptions on A, g, ^ , etc. (cf. [202]) it has been shown that a solution of (7.4.103) exists and is such that: (1) Wt, Wxx belong to L°° (D), (ii) W^ is Hdlder continuous (exponent 1/2) in D (iii) W^ (x, ^) < 0 ioT 0 < X < S (t) ,0 < t < t^, (iv) S{t) is Holder continuous and monotone decreasing, and 5(0) = ^0, S{t) > 0.
7.4.3
The Variational inequality formulation of a two-phase Stefan problem
In the one-phase Stefan problems, the freezing index u{x, t) proved to be very useful as the gradient of u is continuous in Q (the region under consideration). The temperature constraint T > 0 ( < 0) also holds throughout Q. In the two-phase problem although freezing index can be defined, the constraint T > 0 ( < 0 ) does not hold throughout the two-phase region. An approach different from the one adopted for the one-phase problems is required for the variational inequality formulation. If the weak enthalpy formulation is considered for the two-phase Stefan problem, then the enthalpy, which is a multi-valued function of temperature can be written as (for simphcity take specific heats and densities of the two phases to be unity) h{T) = ho{T) + I sgn+{T),
(7.4.105)
where ho{T) is the sensible heat and sgn^ represents Heaviside graph (cf. (4.3.45)). In terms of the freezing index u,
u{x,t) = f T{x,t)dt, {x,t) ent. = nx (0,^*), n c i?^
(7.4.106)
Jo the heat energy equation (4.2.2) can be written as (see [204] for the derivation of (7.4.107)) -ho{ut) + V\ -h hi{x) e I d'^iut), a.e. in Qt^ (7.4.107) Here, hi(x) is the initial enthalpy of the material, and d"^ is the subdifferential of a convex function ^(2) = max (0, z) which coincides with the multivalued mapping sgn^{T). Using the definition of the subdifferential of a function (see (4.3.44), the parabolic variational inequality for the enthalpy formulation can be obtained (cf. [204] and the references mentioned there). Our interest here is in the classical formulation. A variational inequality formulation for a two-phase Stefan problem in the region Q C R^ has been studied in [205]. The phase-change boundary $(t) : t = S(x), x e R^,
192
Elliptic and Parabolic Variational Inequalities
divides Q into two disjoint regions Qi and ^2, representing the solid and the hquid region, respectively. The temperature in the interior 171 of Qi is negative and in the interior 1^2 of ^2 is positive. The cases of regions of zero temperature within Qi and ^2 can also be considered. Initially Q is occupied by ice. On the portion dQ^ of the boundary dQ of Q, temperature b{t) is prescribed and the remaining boundary of ^ is insulated. Let x be a point in Q which is initially at to = 0 in the solid phase, and ti{x) be the first time that X is in the hquid phase, and t2{x) > ti{x) be the next time at which x is again in the sohd phase. In this way a sequence {1^} can be defined such that to(= 0), ti, ^2, • • • , are such that in (^^,^^+1), x is in the liquid (solid) phase if i is odd (even). Since the normal derivative of VT is not continuous throughout Q we define another dependent variable V{x,t) by the equation V(x, t) = Ki r T(x, T)dr + K2 r T(x, T)dT + ... Kj [ T{x, T)dT, JtQ
Jtl
(7.4.108)
JU
where for t E (^^,^^+1), J = 1 (2) if z is even (odd). Ki and K2 are the thermal conductivities of solid and liquid regions, respectively and are taken as constant. Densities of both phases are taken to be equal to unity. If T+ = sup (T, 0) and T~ = sup (—T, 0) where 0 is the phase-change temperature then V{x,t) can be written as V{x,t)=
[' {-KIT'{X,T)
+ K2T+{x,r)]dr.
(7.4.109)
Since T is continuous throughout the interior of Q, dV — = KjT{x,t),
(7.4.110)
where J = 1(2) if z is even (odd) for t ^ {t^, t^^i). We are now interested in obtaining a diff"erential equation for V{x, t) which holds throughout Q. On differentiating (7.4.108), we obtain \/V(x, t) = KA r^"^ VTdr -h T(x, ti{x)) \ + K2\ T '
VTdr+T{x,t2{x))-T{x,ti{x))W...+
KjU'
\/TdT-T
(x,t,)\
. (7.4.111)
At ti{x), ^2(0;),..., ti(x), phase-change takes place and, T{x,ti) = 0, z = 1, 2... Therefore VV[x,t)
= Ky r VTdT + K2 [ ' VTdT + ... Kj [ VTdr. JO
Jti
(7.4.112)
Jt,
Note that V, dV/dt and W are continuous in Vt. VT is discontinuous only across t = t^{x) and not within the interval (^^,^,^1). When (7.4.112) is differentiated, we get VV(x-, t) = K\ r V^TdT + K2 I V^Tdr Jo
Jt]
7.4. Some Variational Inequality Formulations of the Classical Stefan Problems -}-{KiVT{x,ti).Vti{x)
- K2VT{x,ti).mi{x)}
+Kj I V^Tdr - KjVT{x,
193
+ ....
ti).Vti{x).
(7.4.113)
If the Stefan condition (1.4.17) is used in (7.4.113), then we obtain (take pi = P2 = 1) VV(x,0-Ci/
--dT + C2
Jo
—dr-hCj Jti
OT
OT
-—dr^l-l^
(7.4.114)
Ju OT
Performing the integration in (7.4.114) and remembering whether J is odd or even, we get VV(x,t)
-
-CiT(x,0) + / + ^ ^ , K2 ot
=
-CiTix,t)
if T(x,^) > 0 ,
+ ^^,iiT{x,t)<(i.
(7.4.115)
Since T(x,t) has the same sign as dV/dt (see (7.4.110)), (7.4.115) can be written as
vV,M).-c,T(.,o) + ,*,^|+ | [ f ] ^ | [ f ] ;
(7,4ne,
where H{dV/dt)
=
1, if dV/dt
> 0,
H{dV/dt)
= 0, if dV/dt < 0.
In (7.4.116), [x] are the positive and negative parts of x. What happens when dV/dt = 0 or T(x, t) = 0? If there exists a sharp boundary r separating solid and hquid phases at which T — 0, then the meas(r) == 0 in Q and (7.4.116) holds a.e. in Q. Two other cases may arise. For example there may exist regions of non-zero measures in Q.i{t) or 02(^) whose temperatures are zero. It has been argued in [205] that if such a region exists in the solid region then we define H{dV/dt) = 0 when dV/dt = 0 and if such a region exists in the liquid region then we define H{dV/dt) = 1 if dV/dt = 0. In all the three cases (7.4.116) holds in Q. To obtain a variational inequahty for the above problem, the procedure adopted is similar to the one used for obtaining the weak enthalpy formulation in (7.4.107). We write ^ dV dV / / ( ^ ) e a.o(^),
(7.4.117)
where go(x) = x+ defined as sup(0,x) and dgo is the subdifferential of ^o- Now our objective is to look for an appropriate convex function gi{x) : R ^ R so that we can write VV(a:,<) + C , r ( x , 0 ) e % , ( ^ ) .
(7.4.118)
194
Elliptic and Parabolic Variational Inequalities If gi{x) is defined as
ffiW = I (Ci/Ki) {x'f + ^ {C2/K2) (x+)' + lx+,
(7.4.119)
then using the definition of the subgradient of a convex function, i.e., f{x) G dg{x) <=> giO ~ di^) ^ (C ~ ^)/(^) for 3-11 ^ G /?, a relation of the form (7.4.120) can be obtained. The relation (7.4.118) can be expressed as
giiO - ffi(^) > ( W + C,T{x, 0))U-^Y^(e
R.
(7.4.120)
The boundary condition to be satisfied by V{x, t) on dQb can be expressed as V{x,t)
= Ki rb{T)dT-\-.. Jo
=
-^Kj f b{T)dT, on dftb. Jti
J^i^K2{b{r)y-Ki{b{r)y'^dT,
on dQb-
(7.4.121)
On the remaining boundary, we take dV —- = 0, ondn\dnb. on
(7.4.122)
Let the initial condition for V be given by l/(x,0)-0,
xen.
(7.4.123)
On taking ^ = \E^(x), x e ft in (7.4.120) and integrating over Q, we obtain
GW -G(%)>J^
VV (* - f )
(7.4.124)
where (7.4.125) C;(^) = [ gi{^lf{x))dx. Jn If the spaces to which ^ , V and dV/dt belong are defined suitably such as in (7.4.128), then (7.4.124) can be identified with a variational inequality formulation of the above two-phase problem. On integrating by parts the first term on the r.h.s. of (7.4.124) and assuming that ^ and dV/dt satisfy the same boundary conditions, we obtain
where a [V, Z)= f VV. VZ dx. Jn
(7.4.127)
Let M{t) = {Z eH\n),Z\Q^^
= b{t)], a.a. te (0,^.)
b{t) = K2{b{t)y -K,{b(t)y.
(7.4.128) (7.4.129)
7.4. Some Variational Inequality Formulations of the Classical Stefan Problems
195
If there exists a V and the following conditions hold:
1. v,dv/dteL^o,t,;H\n)), 2. dV/dt e M{t) (defined in (7.4.128), 3. The inequality (7.4.126) is satisfied for all ^ G M{t) for a.a. t G (0,^=,), A. V = 0 when t = 0, then V is called a solution of the variational inequality (7.4.126). Under the assumptions that T(x,0) G Z/^ (Q) and b{t) G L'^{0,t^), a unique solution of (7.4.126) exists (cf. [205]). The numerical solution of the above variational inequality can be obtained with the help of a finite-difference scheme.
Chapter 8 The HyperboHc Stefan Problem 8.1
Introduction
A hyperbolic Stefan problem is concerned with a phase-change problem in which the heat energy equation is hyperbohc. The change in the problem formulation introduced by the 'hyperbolic nature of the heat equation' is significant in many ways. The speed of heat propagation in the parabolic case is infinite which can be seen from the temperature solution given in (8.1.1) which is concerned with the problem of one-dimensional heat conduction in an unbounded medium with a constant initial temperature To [24].
For t > 0, howsoever small, and x howsoever large, T{x, t) is different from To which confirms the infinite speed of propagation of heat flow. In the parabolic case, heat flux is governed by the Fourier's law (cf. (1.3.8) and (1.4.9)). Thermal energy is transported by molecular motion which has flnite speed and therefore Fourier's law seems to be a low order approximation to a more general constitutive law. Cattaneo [206] observed that the infinite speed of heat propagation in Fourier's law is due to diffusion and proposed that heat pulses ought to be transmitted by waves at finite but perhaps high speeds. This does not mean that Fourier's law leading to the diffusion equation should be discarded. The relaxation time r in the hyperbohc model is generally very small in nearly all practical and exotic applications. As the heat fiux equation (8.1.3) given below suggests that even on the shortest time scales of our daily lives, we get Fourier's law [207]. The use of hyperbohc heat equation in a mathematical model at very low temperatures is generally accepted, but its use at high temperatures has been debated in the literature and we refer the reader to an exhaustive review article [208] on the 'problem of second sound' (finite speed of heat propagation). Our interest in the hyperbolic Stefan problems in this volume stems from the fact that models of these problems require many interesting physical concepts in addition to those 196
8.1.
The Hyperbolic
Stefan Problem
197
associated with the Stefan problems. For example, in the hyperbolic Stefan problems, both t e m p e r a t u r e and flux can be taken to be discontinuous across the phase-change boundary. T h e delay in the release of latent heat and sensible heat can be considered. These, and several other variations in hyperbolic problems make these problems fairly interesting from the point of view of model and analysis. Many studies of hyperbolic Stefan problems have been conducted. There is still a need to collate the results of these studies in one source.
8.1.1
Relaxation time and relaxation models
In an idealized solid, for example, thermal energy is transported by two different mechanisms: (1) by quantized electronic excitations, which are called free electrons, and (2) by the quanta of lattice vibrations, which are called phonons. These quanta undergo collisions of a dissipative nature, giving rise to thermal resistance in the medium. A relaxation time r is associated with the average communication time between these collisions for the commencement of the resistive flow. In essence r is the relaxation parameter or the time which should elapse for the heat flow to take place after the temperature gradient is formed. There are different times of relaxation, so the mean relaxation time is generally not known. For such sohds, it would be more important to know which modes carry the most heat and so we want the dominant rather than the mean mode of relaxation. How does heat flux respond to the temperature gradient? This has been modelled in the literature in different ways. For example, the flux law in the hyperbolic model has been described as q{t + r ) = -V(KT),
r > 0,
(8.1.2)
where, q is the heat flux vector and r is the relaxation time [209]. In this model, if the temperature gradient is formed at time t then heat is released instantaneously at time t -\- T. T h e first order approximation of (8.1.2) gives rqt + q= - V ( / C T ) , r > 0.
(8.1.3)
This model in (8.1.3) is commonly used in the place of Fourier's law if finite speed of heat propagation is considered. In the relaxation model (8.1.3) a delay of time r in the heat flow is there but the heat release is not instantaneous at time t + T and is distributed over a time interval. Equation (8.1.3) can be derived as a particular case of the following linearized constitutive equation for the heat flux vector q [210]. t
(7{x, t)=
f K{t - r)VT{x, -oo
CXD
r)dr = -
IK{p)VT{t
- p)dp,
(8.1.4)
0
where A' : (0, +oo) -^ i? is a given kernel called the 'heat flux relaxation function' and accounts for the memory of the material. It can be seen that if K{t) = {KQ/T) exp {-t/r)
198
The Hyperbolic Stefan Problem
in (8.1.4), then equation (8.1.3) is obtained. In (8.1.4), the temperature gradient induces heat flow after a delay of time r and heat is released during a period of time, whereas, in the model (8.1.2), heat is released instantaneously after a lapse of time r from the moment temperature gradient is formed. We shall be deahng mostly with the heat flux model given in (8.1.3) which is called a generalized Fourier's law or a non-Fourier's law. Fourier's law cannot be derived from (8.1.4) unless VT is constant for all time and the thermal conductivity is defined as J^K{p)dp. It is clear that the flux law given in (8.1.3) is a particular case of a more general law stated in (8.1.2). Why do we not accept (8.1.2) as the generalized Fourier's law? This question has been discussed in [211] with the help of a simple experiment. Consider a piece of ice of unit volume at temperature T < 0. It is heated by a heat source of constant intensity F > 0. Let Cs and CL be the specific heats of ice and water, respectively. The temperature of ice increases at the rate F/Cs until it reaches T = 0 (melting temperature). The temperature remains at zero for time Csl/F which is also the time taken for I units of heat to be supplied to ice. In the parabolic Stefan problem, the specific energy e{t) (or specific enthalpy) at time t is given by e{t)=P{Tit)) + im-
(8-1.5)
Here, /3{T) = CsT for T < 0, /?(T) = dT for T > 0, .f is the water-fraction and ^ G Hg{T), where Hg is the Heaviside graph (cf. (4.3.45)). To be consistent with the assumption made in the constitutive law (8.1.2), the response of energy to the latent heat should also be delayed and the equation (8.1.5) in the hyperbolic Stefan problem may be written as e{t)=P{T{t))
+ li{t-T).
(8.1.6)
Let us assume that there is no delay in the release of sensible heat so that the energy conservation law pCTt = —V^ holds. In the classical Stefan problem if ice attains the temperature zero at time ^ = 0 and is receiving heat, then it starts storing latent heat for ^ > 0. The relation (8.1.6) suggests that storing of latent heat does not take place during the time internal {t, t + r). Therefore ice does not melt during time (t, t + r) and its temperature goes on rising. At time ^ + r, this 'superheated ice' which has sucked in enough energy suddenly changes to water. This is a very unstable and unrealistic situation. The constitutive equations for the heat flux vector have been considered in much more general form than the one considered in (8.1.4). For example in [210], a general theory of heat conduction with finite wave speeds has been built, in which the heat flux vector is taken as a functional of temperature, temperature gradient and temperature summed histories. Heat flux, like the entropy, is determined by the functional for the free energy. In [212], temperature, temperature gradient and the time rate of temperature are taken as independent variables. When the constitutive equations include rate of temperature, there arises local entropy production in addition to entropy production due to conduction. Within the framework of rate-dependent theory, thermal waves can occur in the finite theory but not in the corresponding linearized theory.
8.2. Model I: Formulation with Temperature continuity at the Interface
8.2
8.2.1
199
Model I : Hyperbolic Stefan Problem with Temperature Continuity at the Interface The mathematical formulation
Several physical and mathematical models have been proposed in the literature for the hyperbolic Stefan problem. In model I, the response of flux to the temperature gradient is delayed but the storage or release of energy as latent heat is not delayed. Temperature is assumed to be continuous and known across the phase-change boundary and heat flux is given by (8.1.3) which is taken as a 'local constitutive law'. The energy conservation equation is derived on the basis of energy conservation law (cf. § 1.4.7) and therefore the energy equation in a medium which is not undergoing phase-change is given by pC^
= -divq,
(8.2,1)
where q is given by (8.1.3). On using (8.1.3), f can be eliminated from (8.2.1) and we obtain rpCTtt + pCTt = KV'^T.
(8.2.2)
Equation (8.2.2) is a hyperbolic equation and is commonly known as telegrapher^s equation. In the one-dimensional case the wave speed of this telegrapher's equation is {k/rY^'^ because the characteristic curves are given by (cf. [12])
and the lines x ± {k/rY^'^t — constant, are the characteristics. A one-dimensional two-phase hyperbolic Stefan problem For simplicity, we first consider a one-dimensional problem in the region $1 = {x : 0 < X <\\. X = S{t), 0 < t < t^, is the phase-change boundary. Let the region 0 < x < S{t) be solid, and the region S{t) < x < 1, be liquid. It will be assumed that in both these regions equations of the type (8.2.2) hold good. The thermal properties and relaxation parameters of the two regions could be different but densities are taken to be equal. Differential equations rspCs{Ts)tt + pCs{Ts)t = KSVTS^
0<X<
S{t), 0
TLpCLiWu + pCLiTL)t = K L V ' T L , S{t) < x < 1, 0 < f < f*.
(8.2.4) (8.2.5)
Initial conditions (i) T(x,0) = To(x), X G Q .
(8.2.6)
200
The Hyperbolic Stefan Problem (ii) ^ix,0)
= go{x), x€Q.
(8.2.7)
On what physical basis can dT/dt at /; = 0 be prescribed? There is no clear answer to this. If q{x, 0) is continuously differentiable in the whole of the region 0, then (8.2.7) can be replaced by (iii) q{x,0) = qo{x), x e Q.
(8.2.8)
Boundary conditions at the fixed boundaries If the temperature is prescribed on the fixed boundary, then the same boundary condition continues in the same form in the present model but if the boundary condition is of the type (1.4.38), i.e., flux is prescribed then we have the following boundary condition on the boundary dQ of f]. q(x,t) = PE{T^-T^)
+ ( C i T - C2T0), on DQ.
(8.2.9)
Boundary conditions at the free boundary (i) Isotherm condition : T = Tm (constant) on x = S{t). Isotherm condition also imphes continuity of temperature. We shall see later that in some formulations temperature could be discontinuous across the free boundary. This is not surprising because hyperbolic equation admits discontinuous solutions. (ii) Energy balance condition : If the phase-change is taking place from hquid to solid, then the energy balance at x = S{t) will have the form pl'^
=
l=l + {Ci-
Cs)T^-
(8.2.10)
Equation (8.2.10) is not suitable for obtaining analytical and numerical solutions as it is in terms of fluxes. So it will be expressed in a different form. On diflferentiating (8.2.10) with respect to time and using (8.2.1) and (8.1.3), we get ;d'^S , , . . dS . ^ , ^ dS P ' ^ = fe),+ ( ? L ) , ^ - ( ? . ) , - ( t o ) . ^ ,
jSs
,^dTs.
^ dn +
ds
dn
,, I (8.2.11)
{Ks-^^q,)lTs OX
I S{t)
If isotherm conditions are differentiated with respect to time, we obtain {TA = - ( T 4 ^ , and ( T ^ = - ( T 4 ^ .
(8.2.12)
8.2. Model I: Formulation with Temperature continuity at the Interface
201
On substituting (8.2.12) in (8.2.11), we to obtain the following equation. £S
/ +
OTL
^ dTs\ [dSy
qjTs-qjTL.o^x
.^^ , ,dTs
,,^ ,
= S(i).
.dT^ (8.2.13)
If Ts = TL, then the last term in (8.2.13) can be written with the help of (8.2.10). For a one-phase problem, the condition (8.2.13) can be easily obtained, for example, take TL = 0, and TS = TL = r. In the above hyperbolic Stefan problem, there are three velocities, one that of the free boundary and two of the wave fronts in the two phases. Can the velocity of the free boundary exceed that of the wave front? This question has been studied in [213] with the help of an analytical solution of a one-phase problem. Consider a semi-infinite solid 0 < a: < oo which is initially in a solid state at the melting temperature Tm- This solid is heated at x = 0 so that we have a one-phase melting problem. It can be easily seen that the pair (5(i),TL), where S{t) = bt, b>0 and b^ ^ k/r,
(8.2.14)
and TL{x,t) = Tm^- (I/CL)
[exp {b{x - bt)/{Tb^ - k)} - l] ,
(8.2.15)
is a solution of the hyperbolic Stefan problem (8.2.16)—(8.2.18). rpCLinh
+ pCL{TL)t = KLV^TL,
0<X<
S{t), 0
TL = Tm, on x = S{t),
,„+{iA)^-(cj()(T.)
f)'-'/^
(8.2.16) (8.2.17)
, on X = S{t).
(8.2.18)
The constant b in (8.2.14) can be determined, for example, from the temperature prescribed condition at x = 0, i.e., TL(0,^) = Tm^ (I/CL)
[exp{bH/{k - rb^)) - l] .
(8.2.19)
We shall now examine three cases: (i) 6^ > k/r, (ii) 6^ < k/r and (iii) 6^ = k/r. Case 1. 6^ < k/r. It can be seen from (8.2.15) that TL > T^ for 0 < x < S{t) and this is a physically realistic case. Case 2. 6^ > k/r. From (8.2.15), Tiix.t) < Tn for 0 < x < S{t) and we have a supercooled water progressing on ice. If (T^)x is calculated from (8.2.15) and substituted in (8.1.3) and the differential equation so obtained is integrated, then we obtain q{x, t) = plb exp [b{x - bt)/(Tb'^ - A;)j , 0 < x < S{t).
(8.2.20)
202
The Hyperbolic Stefan Problem
From (8.2.20), flux is positive and so the heat is flowing from the supercooled water to ice which is not possible as it violates laws of thermodynamics. Therefore the solution in which P > k/r occurs is not physically realistic. Case 3. 6^ = k/r. In this case, equation (8.2.18) cannot be used at the free boundary but (8.2.10) can be used. We shaU try to obtain qj^{S{t),t) occurring in (8.2.10). Prom the first equation in (8.2.12), (TL)^ can be written in terms of (T^)t and from (8.2.1), {T^)t can be expressed in terms of (qi)^. When this (gj^ is substituted in (8.1.3), we get the following equation {QA + HQLI = -QL/T,
-{QL)
on X - S{t),
-h {l/r)qL = 0, on x = S{t).
(8.2.21)
(8.2.22)
The solution of (8.2.22) is given by qi = v4e~^/^, A = arbitrary constant.
(8.2.23)
qL in (8.2.23) does not satisfy the interface condition pldS/dt = qiit)The second law of thermodynamics requires that in any process there should be a positive entropy production. In the case of classical Stefan problem this implies that the condition -T:,{x,t)q{x,t)
>0,
(8.2.24)
must hold at each point of solid and liquid regions. In the classical Stefan problem, the condition (8.2.24) gets satisfied as the flux is defined through the Fourier's law. In the hyperbolic Stefan problem the condition (8.2.24) may not always hold, and an additional condition may have to be imposed to have a physically acceptable solution. If 6^ < r/k, then the condition (8.2.24) is satisfied but not if b^ > r/k. The model I has been used in [214] to obtain a short-time analytical solution of the problem of melting of a semi-infinite solid x > 0 (one-dimensional problem). dTi/dx is prescribed at x = 0 in terms of a 'Dirac delta function'. An analytical solution has been obtained using a suitable Green's function and after making some assumptions about the form of the free boundary, a short-time solution has been obtained. In a multi-dimensional hyperbolic Stefan problem, when the phase-change is taking place from liquid to solid, the energy balance condition at the interface is given by the equation [H]V-n,
= {qs-qL)-n..
(8.2.25)
Here, V is the velocity of the interface and n = {nt.n^) is the unit normal to the interface which points into the liquid (see (1.4.17)) and [^] is the jump in the enthalpy.
8.2. Model I: Formulation with Temperature continuity at the Interface
8.2.2
203
Some existence, uniqueness and well-posedness results
A one-dimensional one-phase hyperbolic Stefan problem Consider the following problem: rqt + KT:, + g = 0, and CpTt + q^ = 0] XQ < x < S{t), S{0) = 0, i > 0, (8.2.26) T(x, t) = 0, and pldS/dt = q{x, t); on x = S{t), t > 0,
(8.2.27)
T{x, 0) - To(x), and q(x, 0) = ^0(2:).
(8.2.28)
The boundary condition could be any of the following type. (i) T{xo,t) = t ( t ) , ^ > 0 .
(8.2.29)
(ii) q{xo,t) = q{t), t > 0.
(8.2.30)
(iii) q{xo, t) = h [f{t) - T(xo, t)] , t > 0.
(8.2.31)
By considering the solution of a specific problem, it was shown in § 8.2.1 that the velocity of the free boundary cannot exceed the wave front velocity. It has been argued in [215] that if | S{0) \ > (/c/r)^/^ then x = S{t) would be a space-like curve for small t. Therefore to have a well-posed problem for {q,T), besides the condition (8.2.27) on X = S{t), one should impose two more conditions on x = S{t) if 5(0) > {k/ryf^ and no condition when S(fS) < {k/rY^'^. Otherwise one would lose either uniqueness or existence of the solution. Depending on the value of XQ, two cases arise. Case I: XQ < 0. The following result has been proved in [215] which is valid for a short time. Proposition 8.2.1. compatibility condition
Let To, qo G C^(-oo,0). Suppose that at {x,t) = (0,0), the
Cp^To(0)go(0) - pl^QoW
(8.2.32)
\qom < pl{k/Ty/\
(8.2.33)
is satisfied and also
Then there exists ^0 > 0 such that in [0, ^0], the problem (8.2.26)—(8.2.28)'with any of the boundary conditions in (8.2.29)-(8.2.31) has a unique solution (T, q, S) G C^xC^xC'^. If To.qoE C°° and C°° compatibihty conditions are satisfied, then the solution G C°° [0, to].
204
The Hyperbolic Stefan Problem
The compatibility condition (8.2.32) can be obtained if the time derivative of the isotherm condition is obtained and the derivatives occurring in it are calculated in terms of the known quantities at x = 0. Condition (8.2.33) imphes that S{t) is less than the wave velocity at least initially. Case II: xo = 0. In this case no initial condition is required. If q{t) and T{t) G C^ [0, oo] and q{xo,t) is determined from (8.2.30) or (8.2.31) and satisfies the condition 0 < g ( 0 , 0 )
(8.2.34)
then in [0, ^o] a unique solution (T, q, S) e C^ xC^ x C^ exists when the boundary condition is either (8.2.30) or (8.2.31). In this case the problem consisting of (8.2.26)-(8.2.28) and (8.2.29) is not well-posed. Global solution for the one-phase problem If the following substitutions are made in (8.2.26)—(8.2.28), then the resulting equations become dimensionless and the thermophysical parameters do not appear in the changed equations. Let, \
S = {Cp/Krf'^S.
(8.2.35)
J
For further discussion, bar over the changed variables will be dropped. For the analysis of hyperbolic Stefan problems it is more convenient to formulate the problems in terms of Riemann invariants A and B which are defined as A = T-\-q, a.nd B = T-q.
(8.2.36)
In terms of Riemann invariants the system of equations (8.2.26)—(8.2.28) and (8.2.29) can be written as At + A,-i-^^{A-B)
=
Bt-B,
=
+ ^{B-A)
0] > , Xo < X < S{t), oj
= 0] \ on x = S{t), S{t) = A{x,t) J
(8.2.37)
{A^B){x,t)
{A 4- B)(xo, t) - 2f(t),
t > 0.
(8.2.38)
(8.2.39)
Let ^(x, 0) = Ao{x), B{x, 0) = Bo{x), S{0) = 0. f(t)
(8.2.40)
Proposition 8.2.2. Assume the following : (i) AQ, BQ G C°^[XO,0], XQ < 0, and e C^[0,oo), (ii) r{t) > 0, A'Q < 0, B'Q < 0, 0 < A(0,0) < 1, and (iii) C°°-
8.2. Model I: Formulation with Temperature continuity at the Interface
205
compatibility conditions are satisfied at (0,0) and at {XQ,0). Under these assumptions, the system (8.2.36)-(8.2.40) has a unique solution (A,B,S) e C^ x 0°^ x C°^ for all t > 0 and the solution satisfies A{S,t)>0,
A^<0,
B:,<0,
0<S{t)
(8.2.41)
From Proposition 8.2.1., a unique solution exists in some interval [0,^o] and if the condition 0 < S{to) < 1 is again satisfied then the time interval in which the solution exists can be extended. It has been proved in [215] that 0 < S{t) < I — S, 6 = S(to) > 0 for any fixed to and so the Proposition 8.2.2 holds. For the proof of Propositions 8.2.1. and 8.2.2., the technique of integration along characteristics and linear iteration has been adopted. A Two-phase problem The following one-dimensional two-phase formulation has been considered in [215] in the regions —oo < x < oo and —XQ < X < XQ. ri{qi)i^K,(T,)^
+ q,
C,p{T,\ + {q,)^
=
0^
=
0^
X < S{t)
(8.2.42)
X > S{t),
(8.2.43)
r2{q2)t + K2{T2)^ + q2 = 0 | C2p{T2\ + {q2)^ = 0 Ti{x,t)
=
plS{t)
=
J
T2(x,t) = 0 ] \ , x = S{t), S{0) = 0, {qi-q2){x,t)j
Ti(x,0) = Tio(x), qi{x,0) = qio{x),
(8.2.44)
x < 0, ;.2.45)
T2(x, 0) = T2o(x), q2{x, 0) = q2o{x),
x > 0.
For the boundary conditions at the fixed boundaries, see (8.2.47). For a local-in-time solution of the above problem, the following proposition holds. Proposition 8.2.3. If Tio, gio, T20, and ^20 ^ ^ ^ ^^e compatibility conditions are satisfied at (0,0) and k2o(0) - ^io(0)| < min {plih/nY^^ then a unique solution (Ti,T2, gi, 92) e C\ exists in some interval [0, ^0], ^o > 0.
^/(V^s)'/'} ,
(8.2.46)
S e C^, of the problem (8.2.42)-(8.2.45)
The compatibility conditions at (0,0) can be derived by calculating the total time derivatives of the isotherm conditions and replacing the partial derivatives so obtained
206
The Hyperbolic Stefan Problem
by the known quantities. The condition (8.2.46) imphes that the velocity of the phasechange interface should be less than the minimum of the two wave velocities in the two phases, atleast initially. For the global-in-time solution, the case TI = T2 and ki = k2 was considered in [215]. The region [—XQ, XQ] , XQ > 0 has been considered. To complete the formulation in addition to (8.2.42) —(8.2.45), at the fixed boundaries, temperatures are prescribed as Ti(-xo,t) - ti(^), t > 0; T2(xo,0 = f2{t), t > 0.
(8.2.47)
Proposition 8.2.4. If Ajo, Bjo, tj, j = 1, 2, are all C°°- functions, C°°- compatibility conditions are satisfied at (0,0) and at (±xo,0), and (_l)-/+i fj > 0, {-iy+'
t ; > 0, J= 1,2,
^Jo < 0, 5jo < 0, ^ = 1,2; and |5(0)| < (/c/r)'/^
(8.2.48)
(8.2.49)
then the two-phase problem considered in [—XQ, XQ] has a unique C°°- solution in (0, 00). AJQ, BJQ are the values of Aj and Bj at t = 0. The Riemann invariants Aj and Bj are defined as Qj = yJ{kj/r){Aj
- Bj), J - 1,2,
(8.2.50)
Tj = yJl/{Cjp){Aj
-h Bj), J = 1, 2.
(8.2.51)
The existence and uniqueness of the solution of a one-phase problem similar to the one described in § 8.2.1 in the region 0 < 2; < S{t), have been discussed in [216]. Using Riemann functions and hyperbolic-equation theory (cf. [216]) the solution of the problem can be expressed in terms of an integral and it can be shown by taking appropriate limits that if at X = 0 any temperature other than the melting temperature is prescribed then the problem is ill-posed. Flux prescribed boundary condition has been considered. The main result of [216] is as follows. Proposition 8.2.5. If g = Lip{Lg, RQ), RQ > 0 and TQ G (0,1), then the one-phase hyperbolic problem with the initial condition /(O) = 0 and f\0) = ro has a unique solution on some interval of time [0, Ri], Ri > 0. The solution can be continued to a maximum interval [0,RQ] such that either RQ —> -fco or min(/'(r), 1 - f'{r)) = 0 as
r^Ro-0. In the above proposition Lip{Lg,Ro) is the class of functions satisfying Lipschitz condition with constant Lq on [0,/?o], f(i/^) = S{t)/(kry^'^ and r is defined by the equation r — / ( r ) = t/r, ro is some value of r, and q is the prescribed flux at x = 0. In addition to other conditions, conditions f{0) = 0 and f'{0) = ro should also be satisfied. The correct choice of ro is dictated by extra physical laws [216].
8.3. Model II: Formulation with Temperature Discontinuity at the Interface
8.3
8.3.1
207
Model II: Formulation with Temperature Discontinuity at the Interface The mathematical formulation
In this model also the response of flux to the temperature gradient is delayed and the response of energy to the latent heat is not delayed but unlike model I, temperature could be discontinuous across the phase-change interface [217]. Consider a one-dimensional problem in which a solid material at time ^ = 0 occupies the region x >0. The melting temperature is taken as T = 0. The constitutive or governing equations are
The boundary condition is taken as f(0,i) =to>0, i>0,
(8.3.2)
and the initial conditions are taken as f{x, 0) = q{x, 0) = 0, x>0.
(8.3.3)
The first equation in (8.3.1) holds in the weak sense as e is discontinuous across the phase-change boundary and the second equation in (8.3.1) is now a constitutive equation and not a localized heat flow law which it was in the model I. e = e{T) is the specific energy (enthalpy) and is defined by the equations (2.1.22)—(2.1.24). The equations (8.3.1)—(8.3.3) can be normalized by making the following substitutions. i=Tt, X = {CVKTY''^I{IP^''^)X,
e - /e,
(8.3.4)
q = {CyKplTY^\, f=^T.
(8.3.5)
On using (8.3.4) and (8.3.5), the two equations in (8.3.1) are transformed into de
dq
^
Bq
dT
,
,
We have e = 1 + T, if T > 0, e = T, if T < 0, and e G (0,1), if T = 0.
(8.3.7)
The initial and boundary conditions become T(x, 0) = q{x, 0) = 0, X > 0; and T(0, t) = fol/Cy = TQ.
(8.3.8)
208
The Hyperbolic Stefan Problem
The hyperbolic system of equations in (8.3.6) are known to have discontinuous solutions (cf. Problem 1.1.6. and [12]) across shocks. If the shock is denoted by x = S{t), then across the shock, the following Rankine-Hugoniot conditions are satisfied. 5(e_ - e+) =q--
q+; and 5(^_ - q+) = T_ - T+.
(8.3.9)
For notations used in (8.3.9), see (8.3.10) and (8.3.11). The first condition in (8.3.9) is the familiar Stefan condition which arises from the conservation of energy at x = S{t). If T_ = T+, then we do not have the second condition at a: = S(t) and we have to necessarily impose the isotherm condition T_ = T+ = T = 0 on x = S{t). (e_,T_,g_)= lim (e,T,^)(x,^)
(8.3.10)
xT5(i)
and (e+, T+, q+) = lim (e, T, q){x, t). xis{t)
(8.3.11)
In a pure solid, e < 0 and in a pure liquid e > 1. It can be seen from (8.3.9) that the shocks propagate with speeds 5 = ± 1 . In the mushy region 0 < e < 1, S = ±0, T_ = T+ = 0 and q- = q+. When phase changes from solid to liquid, i.e., when e_|. < 0 and e_ > 1 or from liquid to sohd {e+ > 1 and e_ < 0) , to pick up appropriate shocks, the admissibility conditions given below should be satisfied [218]. Admissibility conditions A forward shock, 5* > 0, with end states e_ j^ e+ is admissible if and only if T(e) - T{e+) - Ti^zlsili^i^e
- e+) < 0, e+< e < e_,
(8.3.12)
T(e) - T(e_) - Z l £ z l _ Z ( £ ± l ( g - e_) > 0, e_ < e < e+. e_ — e+
(8.3.13)
A backward shock, 5" < 0, with end states e_ ^ e+ is admissible if and only if T(e) - T(e+) - Zl^zl_zZ(£±l(e - e+) > 0, e+ < e < e_,
(8.3.14)
T(e) - T(e_) - ^^^"^
(8.3.15)
^^''^\e
- e_) < 0, e_ < e < e+.
The relevance of (8.3.12)—(8.3.15) in the present problem is that when e„ > 1 and 5" > 0, then necessarily, e^ = T^ = 0 and we have the following one-phase melting problem. Tt + q.^ = 0, and g^ + ^ + T, = 0: 0<x
< S{t), t > 0,
(8.3.16)
8.3. Model II: Formulation with Temperature Discontinuity at the Interface
q{S{tlt) ^ [T(l + T)]'/\S{tltl
and ^
= (-L_y/2^s{tlt),
209
(8.3.17)
T(0,i) = T o > 0 , t > 0 ,
(8.3.18)
(e,T,q){x, t) = (0,0,0), x > S{t), t > 0,
(8.3.19)
e(x, ^) - 1 + T(x, ^), 0 < a: < ^(t), ^ > 0.
(8.3.20)
At this point the necessity of considering the present new model can be questioned. Model I seems to be a good model as it is based on the conservation law of energy and incorporates generalized Fourier's law. As r ^ 0 in the telegrapher's equation and in the Stefan condition (8.2.13) {TS = TL = T), we get back the classical Stefan problem formulation and for r —> 0 even the solution in (8.2.14)—(8.2.19) is the solution of the corresponding Stefan problem. The neessity of considering different models (we shall be considering some more models) for hyperbolic Stefan problems arose from the fact that we should be able to prove the existence and uniqueness of the solution and the well-posedness of the problem under general initial and boundary data and as r -^ 0, the solution of the hyperbolic Stefan problem should converge to the solution of the classical Stefan problem. The solution given in (8.2.14)—(8.2.19) is the solution of a very special type of problem. There does not exist a general result for the hyperbolic phase-change problems which may tell us that as r -^ 0, the solution of a problem formulated with the help of the model I, or for that matter any other model, will tend to the solution of the classical Stefan problem. Different authors have considered different mathematical models, showed the existence and uniqueness of solutions and tried to estabhsh the convergence as r -^ 0 by considering some specific problems. For model I, some results on the existence, uniqueness, and regularity of the solutions of some hyperbolic Stefan problems have been discussed in § 8.2.1 but no result has been reported in [213, 214, 215, 216] on the convergence of the solution as r -^ 0.
8.3.2
The existence and uniqueness of the solution and its convergence as r -^ 0.
The existence of the weak solution of the problem (8.3.16)—(8.3.20) has been discussed in [217] and for this the functions T, q and e considered in 0 < x < S{t) and satisfying (8.3.16)—(8.3.20) are expressed in terms of Riemann invariants (cf. (8.2.36)). {T,q,e) = (0,0,0) for x > S{t). Temperature is prescribed at x = 0. A family of problems parameterized by a time lag r are considered and the desired solution is obtained as T -^ 0. It has been shown that T, q and e defined by Riemann invariants satisfy the boundary conditions and for every simple closed curve C in {0 < x < S{t), t > 0} surrounding a domain R(C), we have / {e{x,t)dx-q{x,t)dt} = 0,
(8.3.21)
210
The Hyperbolic Stefan Problem
f{q{x, t)dx - T{x, t)dt} + / jq{x, i)dxdt = 0.
(8.3.22)
R{C)
The integration along C is taken in the clockwise sense. The Rankine-Hugoniot conditions are satisfied in the sense that for every 0 < ^i < ^2 < oo j ' ' [e[S[t), t)S{t) - q{S{t), t)}dt = 0,
(8.3.23)
and f {q{S{t),t)S{t)
- T{S{t),t)]dt
= 0.
(8.3.24)
ti
The short and long term asymptotics of the solution of (8.3.16)—(8.3.20) were also obtained. The problem considered in [219] is also one-phase with temperature being discontinuous at the interface. The initial and boundary conditions considered are functions of space and time, respectively. In the problem formulation, to start with, energy e{x,t) is taken as e = eoiT) + Z{T)q\
(8.3.25)
where, eo(T) is the classical internal energy as considered in (8.3.7) and Z{T) is defined as
The function Z{T) is a consequence of the second law of thermodynamics combined with the generalized Fourier's law [220]. The coefficients T{T) and K{T) depend both on the temperature and the phase-change material under consideration and they can have jump discontinuities at the phase-change temperature. If it is assumed that r{T)/K{T) is differentiable at T = 0 (zero is the phase-change temperature) and e given in (8.3.25) is substituted in the first equation of (8.3.6), then we have deo
1 d
d TK(T) ')
(r{T)\\
'^ '^'^ 2dT\K{T))]' T^(nn\ \ I ^
(8.3.27)
The r